Thursday, 5 June 2014

Small multiband HF beams- TET-Emtron revisited- 40/20/15/10m 2 element beam

Small multiband HF beams- TET-Emtron revisited

While I haven't posted much recently I have been chasing a few wild ideas.

I bought a few antenna from a deceased estate, including bits of a couple of TET and TET-Emtron HF beams.

TET 2 element 20/15/10 beam

TET 2 element 20/15/10 beams are quite innovative, especially considering they were first made pre-1985 with:
  • capacitive hats to physically shorten the element length
  • dual-drive, driving the radiator and reflector, 180 degrees or more out of phase. This doesn't increase the gain much, but does increase the front to back ratio, which is just as good.
  • close-coupling of radiator and reflector to shorten the boom length.

I had just the radiator of a version of the HB-23M, which is enough to use an antenna analyser to see what it does, albeit at ground level (which messes up capacitive hats and low frequencies). I have to make up the rest.

The traps, the rectangles on the elements, are two parallel R/C circuits tuned to 28 and 21m. Traps effectively short-circuit the antenna at that length. In a full size beam, the 10m trap is at 2500 mm (1/4 wave-length), the 15m trap at 3750mm, the 20m at 7000mm, and the half element/dipole is 10m long for 40m (yes, we are going for a small 40m beam!).

In the TET beam, the inner capacitive hat effectively lengthens the 10/15m section, even though it is only a bit over 2m long. The outer hat is to shorten the element for 20m.

My RigExpert AA-600 analyser, which is really neat, shows resonance at 10 and 25m, but not much at 20m- with the antenna at ground level, which messes up 20m the most. However, others have had trouble with it on 20m (include stuff from Norway.

It would be possible to improve the 20m performance by adding extra length and/or more capacitance. I haven't tried that yet.

OH5IY from Finland has worked with the TET-Emtron antenna: under amateur radio. He has drawn up a HB33M, including the details of the traps.

TET-Emtron 2 element 40/20/15/10 beam, yes, 40m!

When I was last into amateur radio, 40 years ago while at High School (see first post), a retired farmer come amateur had a full-size 40m beam. I have coveted having a 40m beam ever since, with little prospect until coming across bits of a TET-Emtron 40/20/15/10m beam, specifically the radiator main components. I just put them on the old TET mount to see what it would do.

It is common for vertical antenna to use different tricks, traps, loading coils and capacitive hats, to cover all HF bands, including 80m. A vertical is just a half dipole, with the other half reflected through the RF ground radials, which is why radials are important. It is possible to use the same tricks in a beam, just physically more difficult.

The TET-Emtron multiband beam uses 2 traps for 10 and 15m, in the first cylinder in the photo below. The second contains a 20m coaxial trap and a 40m loading coil in the second cylinder. Neat idea, but a fair bit of weight for small gauge aluminium; but it works!

My analyser shows resonance at 40, 20 and 15m, but not much at 10m. Good enough for me!

I had already made a second holder for the reflector. I have a 2m or 4m 50mm boom to mount them on. The next step is to make a reflector, essentially 5% or so longer than reflector. I have the reflector from the HB-23M to use as a guide.

I am contemplating how to do the phasing network, either aluminium, per the TET original, or use coax; the velocity factor shortens the distance needed. Interestingly, I had a newish VHF TV antenna that used the dual-feed; pretty innovative for garden-variety TV antenna- more engineering design than might be expected.

At the moment, I am waiting on parts to build the radiator. One of the most difficult to get parts is thin-wall fibreglass in the right size. A friend suggested a telescopic "squid-pole", so I bought a few; with a view to using them for other antenna- more on that later.

To be continued....


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