Variable-length phasing cables for tuning repeater duplexers: Part 1, Construction of the cable

Variable-length phasing cables for tuning repeater duplexers: Part 1, Construction of the cable


A variable-length phasing cable for use in determining duplexer cable lengths, using M/F SMA connectors to vary the length is proposed. It is a cheaper alternative to a set of phasing cables with a 10mm (5mm?) length increment.

This post describes the concept and how it can be replicated. A simple idea, but some frustration in execution. The basic process of using the variable-length cables is outlined.

The next post will cover the use of the variable length cables and include screenshots from instruments and photos.

The problem: determining cable lengths for inter-connecting duplexer cavities

Determining cable lengths for inter-connecting duplexer cavities is a perennial topic. Notionally, they are a quarter wavelength or odd multiple, allowing for coax velocity factor, but in practice that is a starting point.

My understanding is that commercially, a set of incremental length cables are used to find the correct lengths. Unfortunately, most people don't have such a set of cables.

An additional problem for a six cavity duplexer (3 RX, 2 TX) is that the two cavity inter-connecting cables interact, so both cables need to be varied in length at once to get a good filter response.

A solution: a variable length of coax!

A solution is a variable length of coax. My first attempt was to use M/F N-type connectors with short lengths of cable to change the length in 25mm increments. This was too coarse, giving a 10 MHz or more change in resonance as a quarter wave stub.

My second attempt was to incorporate a home-made sliding trombone joint, made of rolled copper shim for the outer, and small brass tubes for the inner. This gave a continuously variable length of 25mm to fit the gap using M/F N connectors. It works but is too stiff and delicate for practical use.

A variable length of coax using M/F N connectors and a home-made telescopic joint

My third attempt, and the one pursued, was to use SMA M/F connectors to change the cable length. The thin cable is flexible and the SMA connectors give an increment of about 10mm. Effectively, the increment is 5mm as the cable may be too long or too short, adding or removing a connector, where the optimum is half a connector length.

A variable length of coax using M/F SMA and N connectors

The variable length phasing cables are made up from M/F SMA connectors, N to SMA adaptors and SMA M/M pigtail cables. I used cheap Chinese parts to keep costs down; about $2 each. To make one general purpose cable, I suggest 10 M/F SMA connectors and two 100 and 200mm pigtail cables, plus the two SMA-N adapters. Double to make two cables as two cables are needed.

Purchasing the M/F SMA connectors was problematic as they are fairly scarce. More numerous are reverse polarity (RP) connectors. If the description has RP in it, it is the wrong type!

The ordinary purpose of M/F connectors was a mystery to me as the seem to achieve nothing. It seems their main use is as connector protectors on instruments. It seemed such a good idea that I used a few on my instruments. Connectors are only good for about 400 connection cycles, plus the problem of cross threads.

A M/F SMA connector, showing pin. There is a hole in the other end. Most on eBay are reverse polarity, RP types.
Subsequent to building the N type telescopic joint, I found via Google, that such joints are commercially available as an SMA adjustable phase trimmer. Oh to re-invent the wheel, again! But shows my thinking is on track. The only problem with such devices is the cost, about $200 and two are needed. Such a device could be used to give continuous adjustment for the 10 mm gap between the M/F SMA connectors, as per my telescopic N-type joint. However, I have not tried this device and don't know if it is the most suitable; make your own enquires before purchasing!

SMA adjustable phase trimmer

Using variable length phasing cables

While the detailed procedure for using the cables will be covered in another blog post, with photos, the basic procedure to determine duplexer phasing cable lengths is as follows.

1/ Tune each cavity to frequency.

2/ Connect the three cavities with the two same-length adjustable cables, based on a calculation of a quater wave length (or odd multiple), taking into account both the velocity factor of the SMA cable and the length of connectors or Tee pieces on the cavities (assume the connectors are the same velocity factor as the cable). Roughly, 300mm for 2m using RG-214 between Tee connectors on cavities, 12mm less for N-type in-out sockets.

3/ Using a spectrum analyser and tracking generator or a vector network analyser (VNA), take out or insert SMA connectors in both cables until the desired response is achieved. Possibly (to be confirmed in next post), if the desired response is between an increment up or down in length, the desired length is half an SMA connector's effective length, about 5mm.

4/ Using a Tee connector between the ports of the instrument, measure the resonate frequency of one of the correct length variable cables as a quater wave stub. The frequency will be 10 MHz or so above the frequency of the cavity as to cavity connectors contribute to phasing cable length.

5/ Cut a length of cable a little longer than desiered, taking into account any difference with the SMA cable velocity factor. Attach one connector. Connect to Tee and measure the resonate length. Cut the length of the cable to increase the resonate frequency to that of the variable length cable (may need to allow a little for the second connector, to be confirmed, but will be small a couple of mm). If the length is between SMA connector increments, allow the 5mm or so (to be checked).

6/ Attach the second connector and check that the resonate frequency is the same as the variable-length cable. Make a second phasing cable to the same length.

7/ Attach both new phasing cables to the duplexer and check the desired response is achieved.


A variable-length phasing cable for use in determing duplexer cable lengths has been described, as an cheaper alternative to a set of phasing cables with a 10mm length increment. The cable uses M/F SMA connectors to vary the length.

The next post will cover the use of the variable length cables and include screen shots from instruments and photos.