Thursday, 22 December 2016

Building a MMDVM multi-digital mode repeater

Building a MMDVM multi-digital mode repeater- DRAFT 24/2/2017


Very draft. Will write more as I go.

Everything has arrived and I am starting to put it together.

Facebook and Yahoo

How to do everything

_Interfacing the CDM 1250 to MMDVM board.pdf in Yahoo files. For 1.01 board and updated files. (Good but had trouble connecting via the network. Will go back to full desktop)

DMR-UTAH MMDVM XFCE Raspberry Pi Image  *
*        by Chris Andrist, KC7WSU         *
* *
*  Another, includes CDM1550LS code plugs Posts on a number of guides. more guides. mmdvm/MMDVM_homebrew2.pdf

MMDVM hardware MMDVM and Duo Adrino. Arrived! Another source for modem. Bulk order kit.


List of suitable radios:

Radios I have:

MOTOROLA CDM1550LS+ UHF 403-470MHz Mobile Radio, 25W, AAM25RHF9DP6AN. Nice radios but steep learning curve programming them. However, mainly just need frequencies.

TTY MD-390 Seems OK for relatively inexpensive hand-held. I need a DMR TRX to test repeater. Again, programming not simple, but with a couple of others' code plugs, it seems to work ok.

SDR# and DSD+, SDR RX for DMR, DSTAR and other modes. Cheap way to get second RX for testing. Still working on getting it going.

ICOM IC-FR4000 repeater. Was given the repeater and will try to get it working too.

Other bits needed

  • Software- See tutorial- Github. On Windows and written to RPi SDcard.
  • Connectors for TRX- cheap repeater cable for connectors-eBay- arrived. Plugs are 16 pin, TRX are 20 pin. Just use in centre pins, with unused pins on outside on both sides.
  • Programming software for TRX-ether-Downloaded and running.
  • Programming cable for radio-eBay- Arrived. Steep learning curve for Motorola software.
  • Raspberry Pi- I think 2 is easier than 3- see tutorial. Have program on RPi and am contemplating the .ini configuration file
  • Duplexer - I have one that needs to be reduced in frequency. It uses BNC connectors on the cables. I bought some male to female BNC connectors on eBay. Adding one extra connector will electrically lengthen the cable in a simple manner. Should be close enough. Cavities need modifying by solding an extra piece to the resonator, which lows the frequency. Have done one of four and it works fine. They are pass-reject cavities, with a little piston trimmer capacitor to tune notch. Cavity frequency is changed with a screwed capacitive hat.

Tuning modem: Inexpensive tools

Tuning of the modem is covered in page 5 of F5UII's guide. An oscilloscope and spectrum analyser are used. However, many people don't have access to this equipment. However, there may be simpler and/or free ways to do this.

The RX input to the modem is modulated audio. The output to the TX audio that goes straight to the input of the amplifier????

Audio oscilloscope and spectrum analyser software

Many available. Probably work well with high quality 192 kHz USB soundcard. I have Asus Xonar.

Spectrum Lab:

SDR based spectrum analyser and tracking generator $120!

For tuning cavities and possibly modem.

Hotspots DIY


I have bought a DV4mini to get access to didital radio networks via handheld.


Seems ok, but not sure on DMR (via DMR+).
Access to Brandmeister seems limited?
Not pursued much, mainly interested in repeater.

Official? DMR sites Recently appeared??

Online streaming

Hose line is an online streaming platform for the HAM radio Digital Mobile Radio (DMR) Brandmeister network.

Handy to see who is active.

Looks useful

Sunday, 16 October 2016

Using notch filters for Rx and TX and extending the range of a notch duplexer

Using notch filters for Rx and TX and extending the range of a notch duplexer (Draft)

I have a six cavity notch duplexer for 70 cm. At the original frequencies, outside the amateur band, the RX and TX notch responses are a mirror of the other, as notch filter responses are asymmetrical, compared to symetrical for band pass..

The mirroring of the responses are achieved by adding a quarter wave line to each cavity in the TX half. Being a quarter wave length, it changes with frequency when the duplexer was tuned about 20 MHz lower from its original frequencies. This was enough to stop the mirroring, making the RX and TX curves much the same; high losses for TX.

The desired TX shape can be restored by adding about 10 mm to the quarter wave lines on the TX side. The length was calculated as the difference in quater wave between the two TX frequencies, in this case about 8 mm. For this duplexer, it was quite easy as the join to the cavity was soldered. The cable was lengthened by adding a 10 mm piece of coax with the cover and shield removed. The join is wrapped with insulation tape. The shield was made from a strip of copper shim wrapped around the cable and soldered to the other cables shield.

The duplexer RX cans on the right, TX cans on the left.

The RX cans showing a simple T join on the coupling loop.

The TX cans showing the extended quarter wave link between the T and the coupling loop.

The normal RX response with the low-loss on the low frequency side. The T is connected directly to the cavity.
The mirrored TX as a result of the quarter wave loop. The low loss side is now on the high frequency side and the High loss on the low frequency side.

Tuesday, 4 October 2016

DVB-T TX Filter: 23 cm filter using 70 cm duplexer

DVB-T TX Filter: 23 cm filter using 70 cm duplexer


I came across a commercial UHF bandpass cavity filter that was as long as a VHF cavity. I removed a coupling to see inside. The probe was nearly 500 mm long, rather than the 170 mm in most UHF filters. They were using the probe as a three quarter length, rather than the typical quarter wave length. I checked the cavity at VHF and, sure enough, there was another peak.

The obvious question then was whether the 70 cm duplexer filter would have another bandpass at three times the frequency, about 23 cm? It does!

With careful tuning it should be possible to use the ubiquitous 70 cm duplexer at 23 cm.


Raw response

With the filter set to 7 MHz at 446.5 MHz, I had scanned three times that frequency and a very wide span. Sure enough, there was a passband, although at a different frequency and three times wider.

The frequency of these cavity tuners are determined by a fixed length probe (tube) and a capacitive screw adjustment.  The relationship between the probe and the adjustment screw is probably affected by frequency.

The bandwidth is three times greater as the upper and lower frequencies are multiplied by three.


With a bit of tuning, an acceptable 7 MHz filter at 1290 MHz can be achieved. The losses across the channel are a little high but there is usually reserve TX drive. The notches are about -50 dB, which should be adequate as a TX filter.


The general idea of using the filter at three times its normal frequency works. However, the duplexer needs adjust to achieve the desired frequency, bandwidth and notch depth.

I am happy to just measure the performance at the moment. I have a Darko 23 cm amplifier but it is not attached to a heatsink.

It may be better to modify a common 900 MHz duplexer by reducing the length of the tubes, but it is a fair amount of work to pull them apart.


A notch duplexer can be used at three times its nominal frequency as the probes act as three quarter rather than one quarter "antennas".

Monday, 3 October 2016

Duplexer DVB-T TX filter:Technical details (Draft)

Duplexer DVB-T TX filter: Technical details



Bandpass is addition of two notches

 Three cavities RX to ANT

Reverseded notch TX to ANT

Sum of two notches with six cavities RX to TX

Two left hand couplings are conventional, the two right hand couplings, with plastic spacers are unusual, give reverse notch (not sure how).
The image shows internal construction of 4 cavity duplexer. Tuning is achieved by slug into the top of the probes, like a capacitive hat on an antenna. Tubes or probes are slightly less than a quarter wave length. Basically antennas in a box with couplings in and out. Ratio of tube diameter to outside diameter determines impedence. One to three gives about 50 Ohm.

SWR of Duplexer filter

7 MHz on 70 cm, not great but probably affected by tuning, possibly by coupling design.


Coupling for reversed notch needs further investigation

Friday, 30 September 2016

Duplexer DVB-T TX filter: Low bandwidths

Duplexer DVB-T TX filter; Low bandwidths: 2 and 1 MHz.


The duplexer DVB-T TX filter was adjusted to see if it would work at low bandwidths, specifically 2 and 1 MHz at 70 cm. It achieved this easily, but with a small increase in losses.


The duplexer was original adjusted for a 7 MHz bandwidth with a spectrum analyser and tracking generator. I only moved the lower frequency notch, one side of the duplexer.

At the request of a USA operator, wanting to work DX, I readjusted the filter for a 2 MHz bandwidth. This was possible, with with little effect on losses.

For interest, I adjusted it to a 1 MHz bandwidth, again possible, but with slight losses.


The filter works surprising well at the lower bandwidths. I thought losses may have been higher.

The losses are not a major issue as the DVB-T amplifier can be driven a little harder to make up for them, and possible even more because of the filter.

Notch cavity filters could be used, in principle, for DVB-T in other bands as such filters are relatively common, although less so with the notch going both ways. I will discuss this in another post.

Tuesday, 27 September 2016

Testing a duplexer as a DVB-T TX filter-Wow!

Testing a cheap Chinese duplexer as a 70cm DATV DVB-T 7 MHz TX filter- Wow!


In my last post I described a cheap Chinese duplexer re-tuned as 70 cm DATV DVB-T 7 MHz TX filter. The duplexer uses notch cavity filters, six in all. The notch filters have a much sharper edge, compared to a band-pass filter. The sharp notch seems suited to the vertical edges of a DVB-T signal.

I initially check the signal source, a HiDes camera with direct DVB-T output at 1080P. I was a little surprised at the spread, but the filter cleaned it up well. This would indicate the need for a filter before the main power amplifier.

I pressed on with just one filter and tried it at the output of the amplifier, a 10 W device, from Darko OE7DBH, using a RA60H4047M1 60 W module. Even with the indifferent input, the filter was able to reduce the spread to -60 dB and give a clean 10 W output.

The notch duplexer/filter seems to overcome some of the major hurdles with DVB-T amplifiers and warrants further investigation.

I have not investigated the effects of the filter and different power levels on signal quality at the receiver. I have limited instrumentation, but will report my findings in the next post.


A 20 dB directional coupler, plus an additional 30 dB of attenuation, was used to tap a signal from the transmit path to a HP 8591A spectrum analyser. A cheap SWR/Power meter was used in line to give some idea of output.

Filter before the amplifier

The duplexer/filter works remarkably well. In setting up to test the amplifier, I checked the source from a HiDes camera with direct DVB-T output; 7MHz channel, centered on 446.5 MHz, just to check it was clean. It wasn't too good, acceptable maybe at -40 dB, but with quite a spread.

Inserting the duplexer/filter cleaned it up almost perfectly!

Now I need another duplexer/filter to put after the amplifier.

Preliminary tests with just output filter

With the amount of gear needed to test a DVB-T amplifier, I thought I might see how the filter works, even with the less than perfect signal from the camera source.

With no filter and adjusting the input to keep spread at about -30 dB gives about 6 W, but it is not pretty.

Filter after the amplifier

With the filter after the amplfier, the results are surprising; with adjustment, a clean 10 W signal. The spread is 60 dB down. Magic! Drawing about 8 A at 13.8 V.

The power meter is showing 10 W. However I am not sure that is the full envelope power of a 7 MHz wide DVB-T signal. Cheap meters are for measuring low bandwidth CW and SSB signals, not complex ones. (I would like this clarified/explained by someone in the Yahoo group).

For amusement, I tried the direct signal input, thus over-driving the amplifier and putting out a few extra Watts of power.

The result shows the notch-nature of the cavities and duplexer. There is some rubbish, still -40 dB, above and below the filter's two notches.

Taking out the filter, and about 20 W output, with terrible spread. It sure does some clean-up job!


The notch duplexer/filter seems to overcome many of the problems of amplifiers for DVB-T.

A filter seems to be needed both before and after the main power amplifier.

There is still probably a need for a lowpass filter to stop harmonics.

The received signal quality needs to be checked.

This is a preliminary study and needs to be tried by others.

I again thank Martin VK4JVC for suggesting a duplexer rather than building an interdigital filter.

Monday, 26 September 2016

A 70cm DATV TX filter using a cheap Chinese duplexer

A 70cm DATV DVB-T 7 MHz band-pass filter using a cheap Chinese duplexer


DATV transmitters for DVB-T are notorious for "spread" outside the channel, to the point that keeping it 30 dB or more below the signal becomes a limit for power output, typically 10 W out of a 70 W module amplifier.

Even with -30 dB spread, it is desirable to have a band-pass filter before further amplification or transmission. Usually an interdigital filter is used, but they are either expensive to buy or a bit difficult to build.

VK4JVC suggested using a cavity filter duplexer instead. I tried a four cavity notch duplexer, but the pass-band losses were too high, more than 20 dB. I had bought a cheap, ~A$100, Chinese Jiesai duplexer, but had put it aside as the response looked bad. After try other duplexers (notch and pass-reject types), I tried the Chinese one again, this time successfully.

The result is that the Chinese filter seems to provide a good pass-band for the 7 MHz DVB-T DATV signal with acceptable pass-band losses and steep skirts. The next test is to try it with my 10 W amplifier from Darko in Austria.

The filter

The duplexer is a typical mobile device available on eBay for about A$100 delivered, taking a week or so to Australia. The store insisted I supply some tuning data, even though I was immediately going to change it. I specified 440.5 and 446.5 MHz to keep them happy. It came with notches at those frequencies, but had not been well tuned.

The filter has three square cavities for each of RX and TX. The only adjustment is a screw at the top that capacitively alters the cavity's resonate frequency. There are no other adjustments. Each cavity has an cable in and out, but they seem to be notch filters rather than pass-band.

I am not sure what the power handling capacity is, 25 W, from memory. That would make it an adequate final TX filter for most DATV applications.


The Australian 70 cm DATV band is 7 MHz wide centered on 446.5 MHz, with edges at 443 and 450 MHz. I use the whole 7 MHz as it makes reception on conventional TVs easier and I want high quality 1080p.

Using a HP 8591A spectrum analyser and tracking generator, re-tuning is quite easy. Three cavities at a time first (RX-ANT, TX-ANT), then checking all six with the input and output through the RX and TX connectors, ignoring the antenna connector.

I have tuned the cavities to about -3 dB at the channel edges. The loss through all six cavities is about 2 dB which indicates quite reasonable construction. The side slopes are quite steep.

I am not sure this tune will be adequate to suppress the channel spread, but I will re-tune to find an acceptable compromise. I may need to narrow the width, but without affecting the TX signal.


A cheap Chinese duplexer has bee re-tuned to produce what appears to be an acceptable TX band-pass filter for a 7 MHz wide channel on 70 cm.

Despite their reputation, the Chinese duplexer seems of reasonable quality and has typical characteristics for this type of device. The original tuning was indifferent, so should always be checked.

Friday, 15 April 2016

Airspy, Spectrumspy, noise source and UHF cavity filter characteristics; a low cost spectrum analyzer?

Airspy, SpectrumSpy, noise source and UHF cavity filter characteristics; a low cost spectrum analyzer?


A basic spectrum analyzer/tracking generator for less than $250? Yes. Can it be used to do a demanding task like tuning a UHF cavity filter from a repeater? Seems so.

The "proof of concept", spectrum analyzer software, SpectrumSpy, can be used with the Airspy SDR and a noise source to show the characteristics of a pass-reject UHF cavity filter.

SpectrumSpy and Airspy

SpectrumSpy, "proof of concept", spectrum analyzer software is a new addition to the SDR# download for use with the Airspy SDR. It has the potential for a new direction with low cost SDRs, spectrum analyzers.  Spectrum analyzers are expensive; $1500 then skyward. SpectrumSpy: (separate executable in SDR# folder).

Airspy has a 24 – 1800 MHz native RX range, but down to DC with the SpyVerter option. $199  and US$59

An earlier post is of SpectrumSpy used as a spectrum analyser:

However, a spectrum analyzer needs a tracking generator to be really useful and to test radio filters. A noise source can be used to achieve much the same purpose.

Noise source

A low cost  Zenar diode based noise source is available for about $20. It uses three Mini-Circuits ERA-5+ wideband amplifiers (DC-4 GHz) to get the noise to a usable level.

I did a quick check of the white noise with SpectrumSpy right up to 1 GHz. The level dropped a little with frequency, but that could be either the noise source or Airspy. and many others.

Noise source output

Cavity filter characteristics 

A cavity filter is part of a duplexer that allows a radio repeater to simultaneously transmit and receive with the same antenna, an amazing feat in itself and one of my other interests. Duplexers are fairly complex in design and setup.

Pass/reject is one type of cavity filter that passes the receive signal on one frequency, but rejects the re-transmit of the repeater on another frequency with a notch for the receive side of the duplexer and the opposite for the transmit side.

An expensive spectrum analyzer/tracking generator is needed to adjust the pass and reject frequencies and to minimize losses.

However, SpectrumSpy with a noise source does a pretty good job. The shape, frequencies and depth of notch (>30 dB) are about right in this very preliminary test.

Characteristics of single cavity filter using SpectrumSpy and noise source

Spectrum analyzer/tracking generator plot of similar but different filter.

Typical characteristics from half a duplexer (two cavities) with my old HP spectrum analyzer and tracking generator ($12K in 1990s and about $1000 secondhand now). (I will do it for the same filter another time and edit.) It has different tuning to the one in this post, hence the mirrored shape.


Airspy with SpectrumSpy does a good job in this demanding task. It probably could be used to tune a duplexer; amazing for the very low cost.

Two immediate issues however. First, SpectrumSpy is not calibrated or necessarily linear. Second, it does not have the bells and whistles of a spectrum analyzer such as digital analysis and data, or amplitude/frequency markers.

SpectrumSpy is proof of concept of an SDR used as a spectrum analyzer. This is a very good additional application for low cost SDRs, normally used as receivers. Similar software could be developed for other SDRs, as has been done, but less successfully, for the RTL-SDRs.

Thursday, 14 April 2016

Low cost spectrum analyser/ scanner with AirSpy and RTLSDR

Low cost spectrum analyser/scanner software for the Airspy and RTL-SDR


It is not often I am amazed at new technology, especially for free, but the Spectrum Spy software, a spectrum analyser/scanner for the Airspy SDR, impressed me. It is a poor man's spectrum analyser.

However, it is preceded by at least two spectrum scanners for the RTL-SDR hardware; rtl_power and RTLSDR Scanner.

This post will compare the two devices and three software packages, scanning the entire FM band and the 100 MHz of the local TV band.

The software

The three programs all run under Windows, Windows 10 in my case. All three installed and ran without much difficulty.

Spectrum Spy is part of the SDR# software package. It is a separate program to SDR#, but in the same folder. Spectrum Spy has a spectrum and a waterfall. It updates every few milliseconds, depending on the span. Spectrum Spy only works with the US$199 Airspy. I have a V1 Airspy.

RTLSDR-Scanner is a stand-alone program; Use the "setup" version and it will download all the code it needs to install. No extra files are needed for the RTLSDR, provided its driver is installed. RTLSDR-Scanner is a single pass with spectrum but no waterfall.

Rtl_power is a small program that has both spectrum and waterfall. Some additional RTLSDR files are needed to be copied to the program directory.

The hardware: apples and oranges

The Airspy and RTL-SDR both use the same tuner, but after that the Airspy is much more sophisticated. A significant difference is the frequency span, 10 MHz for Airspay and about 1.2 MHz for the RTL-SDR. The span makes a big difference to the scan rate of the scanner/analyser programs.

The question is whether the $20 RTL-SDR is useful as a scanner, compared to the Airspy.

FM band

A handy source of signals is the local FM band. The screen shots show how the three programs perform.

Spectrum Spy performs very well, showing a 20 MHz span. It shows both spectrum and waterfall. The waterfall is very useful for intermittent signals.

RTLSDR Scanner does a reasonable and useful job. The spectrum is comparable with Spectrum Spy. However, it only does a single scan and takes 20 seconds of so for the scan.

RTL_power had an indifferent result. The spectrum is quite different to the other two programs. While it does a continuous scan, the waterfall does not align with the spectrum.

TV band

The local DVB-T TV is in a 100 MHz band from 600 MHz. Displaying the TV stations is quite a test for a spectrum analyser.

The Spectrum Spy does a very good display in this demanding task.

The RTLSDR Scanner similarly does well in displaying the spectrum, although it takes quite some time to do the scan.

RTL_power does better with the TV than the FM, but is still not great.


The Spectrum Spy program with the Airspy hardware does an awesome job comparable to some spectrum analysers, for low cost,

It would be ideal for doing repeater site surveys, especially with the waterfall as well as the spectrum.

RTLSDR Scanner could be used in a similar manner to Spectrum Spy, just taking longer with a single scan.

Saturday, 5 March 2016

200W DATV power amplifier- 50V 20A power supply

200W DATV power amplifier- 50V 20A power supply


I am builder a DATV power amplifier, about 200W maximum. The power supply is unusually powerful as the amplifiers are only bout 25 per cent efficient. Commissioning such a power supply is not simple.

DATV power amplifier

I discovered through one of the Yahoo groups that it was possible to buy UHF DVB-T pallet amplifiers, broad band from 470 MHz to 900 MHz. I bought one through the Italian eBay for 400 Euro. It has a pair of BLF888A LDMOS transistors. The pallet is a pair of amplifiers with a splitter and combiner to allow single in and out. The BLF888A are each a match pair of transistors in push-pull.

Power supply

The power requirements are about 48V at 20A as the amplifiers are only about 25 percent with the ultra linear DVB-T.

I remembered that a lot of telecommunications equipment use 48V, so had a look on eBay. I managed to buy a new ELTEK Flatpack2 2000W 48V HE Power Supply for $150. These are state of the art telecom power supplies and very small for 2000W output.

The first complication is connecting wires to them. They are designed for an in-chassis slot connection of both mains and DC. I opened it up and soldered wires to the back of the connector. Not a good idea, but will do for the moment. I will make up a slot connector and have no wiring inside the power supply.

The next problem is testing the power supply. I bought four 12V headlight globes and mounted them together with wiring. The globes represent a load of about 220 W. The power supply easily achieved this. The voltage is 53V and is not easy to reduce to 48V, it has a CAN network for control.


For the first part of this project, the power supply, it seems to be working well. Next the heat sinking and testing the amplifier, then some filtering.

Monday, 15 February 2016

Three cavity notch RX filter for 2 m with 1.6 MHz RX/TX spacing

Three cavity notch RX filter for 2 m repeater with 1.6 MHz RX/TX spacing


A three cavity notch filter was constructed and tested. The RX filter has over 80 dB of TX rejection.


The club has a 2 m repeater that is to operate on the new 1.6 MHz RX/TX spacing, as opposed to the usual 600 kHz spacing. The existing high/low pass reject cavities could not accommodate the wider spacing, limited to about a 1.1 MHz spacing.

The club has five 1968 band pass filters, old but well made. A pair of them had been used with a phasing harness to act as notch filters, but the rejection was not high enough for TX reject on RX. Two are probably adequate for the TX filter to reject spurious noise at the RX frequency.

I decided to try and make a three cavity notch RX filter, necessitating a new phasing harness. Each of the cables are a quarter wavelength, with those going to the cavities a little shorter to allow for the probe inside the cavity. I used LMR-400 ultraflex for the double screening, plus I had some at hand. RG-214 is preferred but it is very expensive (~$25 per m).

My first attempt was to use soldered tee joins, but I had trouble with shorts. I think the aluminium Mylar film melts when soldering the braid. I changed to N connectors and tee joiners. That was worked.


The RX filter gives over 80 dB TX reject, which is adequate.

I also measured the SWR to see where the minimum was, expecting it to be at the TX frequency, however it is quite some distance away. Previously I had thought that the RX and TX filters would need separate RX and TX antenna, as I thought the RX notch would be a short-circuit for the TX, but the SWR measurement suggests otherwise. I need to investigate this further.


An effective repeater RX notch filter can be made from three band pass cavities and a phasing harness. Further work is needed to see if a common antenna can be used.

Sunday, 14 February 2016

Duplexer isolation: limits of instrumentation

Duplexer isolation: limits of instrumentation


With high isolation duplexers, about 100 dB, conventional spectrum analysers and tracking generators are beyond their capability of about 80 dB. The rejection of a high isolation duplexer can be checked using a RF signal generator and spectrum analyser.

High isolation duplexers.

I have a couple of sets of four cavity 70 cm pass reject duplexers. The two cavities give about 70 dB rejection, which is ok but more would be better for RX. A third cavity can be added for greater isolation.

Each cavity is individually tuned with a spectrum analyser and tracking generator. The two cavity duplexer can then be assembled and tested. A nice plot with about 70 dB rejection at 438.1 MHz and a pass of 433.1 MHz.

Adding the third cavity results in noise at the rejection notch. The spectrum analyser cannot show the full depth of the notch; it does not have enough dynamic range.

RF generator and spectrum analyser

A RF generator can be used instead of a tracking generator as it has a greater output at the TX or reject frequency.

When connected with two cavities there is quite a spike.

 However, when the third cavity is added the spike almost disappears with the extra 35 db rejection.


A RF generator can supplement a spectrum analyser and tracking generator when aligning high isolation duplexers.

Sunday, 31 January 2016

Tuning the receiver front-end of a Kenwood TKR-750 repeater

Tuning the receiver front-end of a Kenwood TKR-750 repeater


I bought a second-hand Kenwood TKR-750-1 VHF repeater. However it was not clear what band the receiver was tuned to. The actual receiver and transmit frequencies can be programmed with a computer. However the the receiver band width is tuned with a spectrum analyser and tracking generator.

This post describes how to re-tune the receiver front end.

The band pass filters for a 2m repeater RX are quite critical as they are the only selectivity. With a 600 kHz spacing, the cavity filters are usually just isolating the TX and RX when sharing the same antenna, but providing no bandpass to strong nearby signals.

Starting point and instrumentation

I have an old HP8591A spectrum analyser and tracking generator, circa the 1990s, but still a capable instrument. The basic procedure is to connect the tracking generator output to the RX antenna input and the spectrum analyser input to a test point after the RX bandpass filters and amplifiers, just before the first mixer.

The pass-band of the receiver will then be shown. Before adjustment, the centre of the pass-band was about 160 MHz and the bandwidth about 10 MHz at 6 dB. This pass-band would make the receiver useless for the 2m amateur band, in Australia, 144 to 148 MHz. I was apprehensive if I could move it that far.

The receiver front end is relatively simple. From the antenna through two filters L2 and L3, a single RF amplifier bipolar transistor, then three more filters, L5, L6 and L7. The test point is CN1, at the bottom left.

The HP8591A is connected to the antenna and test point. However, before connecting the instrument, the output of the tracking generator must be reduced to about -30 dB. It took some searching through HP manuals to see how to do that, but it is easy when you know how.

Just in case I triggered the repeater, I connected a 50 Ohm dummy load to the TX antenna output.

The test setup shows the devices connected together. The five filters can be seen clearly.

Amateur radios use an odd connector, TMP from Taiko Denki, for interconnecting boards and for test points. Fortunately I had chased some up for when I did the IF tap on my IC-7410 TRX. They are $1-50 from the USA, so I bought ten.

The other must have is a RF coax connector adapter kit. The gold connectors going from BNC to N. and They are a must-have for working with modern radios, from HF to microwave. The two connector ends screw together with a threaded barrel.

The adjustment was suck it and see, not having done it before. Using an insulated tuning stick, I started with the first filter, screwing it in as I wanted to lower the frequency. This adjustment showed up nicely on the spectrum analyser. I then went to the next in the chain and so on. I needed to change the centre frequency on the analyser so I could see where I needed to go. I went though the chain of filters trying to get the right shape, right frequency and minimal losses.

The end result: success

Eventually I had a nice pass-band centred on 147 MHz with the same 10 MHz bandwidth, pretty much the same shape as the original. I was relieved I was able to manage to move it sufficiently, otherwise the repeater was of no use.

RX selectivity and cavity filters

The main purpose of a duplexer is to separate the RX and TX to allow them to share the same antenna. As a reader of my blog will see my interest in building cavity filters. I found it quite fascinating that a repeater could transmit 50 W into the same antenna where it was simultaneously receiving micro Watts of RX signal.

On 2m, the spacing is only 600 kHz (recently increased to 1.6 MHz in Australia, but that is another problematic story). Bandpass cavity filters do not have enough selectivity for this narrow spacing. Special pass reject cavity filters must be used. The TX bank of three cavities pass the TX signal but have a deep notch at the RX frequency. Similarly, the RX bank pass the RX frequency but have a deep notch at the TX frequency.

However, while the duplexer allows the repeater to work, the duplexer is not a bandpass filter for the RX. All the selectivity is left to the little filters in the RX itself, making them critical if there are strong nearby signals.

In Australia, there is a pager band starting at 148 MHz, right next to the 2m repeaters.

On UHF, 70 cm, the spacing is higher and allow the use of simpler bandpass cavity filters. These high Q filters do the two tasks of providing selectivity as well as RX-TX isolation.


It is realtively easy to retune the RX frontend of a repeater using a spectrum analyser and attenuated tracking generator. Tuning the repeater RX on bands with close RX-TX spacing is quite critical as it is usually the only bandpass selectivity.

PS UHF TKR-850-1

Before; about 460 MHz centre, 8 MHz wide

After: OK but not quite as good, centre 434 MHz and 10 MHz wide; a 26 MHz move!