Gamma match - adjustment?

[SiriusHardware]

I have a 'Crossbow' type folding / handheld 2-ele 2m beam made by Sandpiper.

Just giving it a check over last night I noticed that the SWR is higher in the upper part of the band and still decreasing at 144.000, so obviously the lowest SWR must be somewhere below that frequency.

The aerial uses the common Gamma match arrangement in which the coax inner turns through 90 degrees and into an insulated sleeve which runs up inside a 6-7 inch element which is close to and parallel to one of the main dipole arms. Near the far end of this is a shorting bar which connects the element to the main dipole arm.

My question is, to adjust the SWR, do I need to change the position of the shorting bar, or do I need to change the position of the 6-7 inch element relative to the centre boom? The shorting bar is screwed into position, so not easy to move.

[Radio Wrangler]

The shorting bar should scale the impedance presented (mostly). The position of the minimum vswr is going to be mostly affected by element lengths. These things all interact but if you iterate with element lengths for frequency of null and gamma tap for depth of null, you ought to converge.

The brass barrels and clamp screws nicked from a suitably sized choc block connector can be used to increase element lengths by an adjustable amount up to several mm.

David

[SiriusHardware]

Thanks David - as the SWR is currently lowest at the low edge of the band I would think that if anything the elements need to be shortened in order to raise the frequency at which lowest SWR occurs (rather than lengthened), but I think I will leave well alone. The antenna was designed for hilltopping and I suppose probably meant to be optimised at the SSB calling frequency 144.300. I've worn out several of these over the years but when I last asked, Sandpiper weren't making them any more.

I will try removing the existing shorting bar, the position of which is determined by screw holes in elements and therefore fixed, and try making a temporary alternative which I can slide back and forth a bit.

Do I take it that the sleeved / insulated input connection is really just a poor man's variable capacitor, the value of which is determined by the position of the shorting bar? I have to admit that when I first started looking at the design, my brain did a bit of a backflip when I realised that the incoming signal from the coax was connected to nothing.. It just seems wrong.

[Radio Wrangler]

It shows the power of capacitance!

It's a funny arrangement, more adjustment of the length of a short transmission line.

DAvid

[Terry_VK5TM]

Originally Posted by SiriusHardware

My question is, to adjust the SWR, do I need to change the position of the shorting bar, or do I need to change the position of the 6-7 inch element relative to the centre boom?

You move the shorting bar to adjust the SWR.


The sliding gamma match is/was a fairly common way of adjusting antenna's.


Making the shorting bar adjustable rather than with set screw down spots makes for finer adjustments.

[SiriusHardware]

I'll see if I can find a couple of metal 'P' clips of appropriate inside diameter to make an alternative shorting bar with. I used the aerial (unadjusted) with my old FT-290R2 last night and it worked very well, but the SWR on the FM portion of the band is a little bit higher than I would prefer.

[Malcolm T]

Currently i am experimenting with a HF Ground plane with gamma match , works a treat on 17mtr band
and increasing the vertical element length about a meter and re adjusting also on 20mts.
Four radials only approx 1 meter off the ground. The base of the vertical element is about 2 meters off the ground . As VK5TM says its all about adjustment of the cap and shorting bar , watching the swr dip on the laptop (VNA) its satisfaction when it drops right into the correct frequency range .
Almost flat across both 17 and 20 mtrs bands.
The capacitor is a large space vane type probably about 150pf and quite few years old.
Also have a 144mhz 3 element gamma match yagi experiment as well with the RG213 inside the shorting bar tube for capacitance , that also has proved a success but now need to increase the element number . Fascinating stuff Aerial physics.
Slowly coming together for going /P

[Malcolm T]

Just bear this in mind for element adjustment lengths, when experimenting with aluminum round tubes, If the smaller tube is not of sufficient clearance inside the larger one there is a very real possibility it will hang up with friction or a grain of sand / grit , it will be impossible to separate them. I found this out the hard way !.
The trick is to find tubes with the correct nominal diameters that aren't too sloppy or too snug fit.
The key word is Adjustable .

[SiriusHardware]

While I agree with all of the above, the aerial which started off the thread is a commercially made item so I am clinging to the hope that they got the element dimensions right. They were obviously built to a fixed design, hence the fixed position of the shorting bar.

Between the two options - shortening the elements, or playing with the position of the shorting bar, moving the shorting bar seems to be the easier and more reversible option to try.

[Techman]

Originally Posted by SiriusHardware

I have to admit that when I first started looking at the design, my brain did a bit of a backflip when I realised that the incoming signal from the coax was connected to nothing.. It just seems wrong.

You're not the only one!

I was given a home-made UHF one several years ago by someone as a gift with some other bits for helping them out with some radio problems. He hadn't made it himself, but was given it along with some other bits by the person that had - or something along those lines...it was a while ago. He told me at the time that it worked really well, although I'm not sure whether he'd actually used it himself or was just saying what its original constructor had told him. It had been in his shed for some time and had damage consistent with probably being trodden on and had one of the reflector (longest) elements broken off. I didn't do anything with it other than tape the broken part to it so that it wouldn't get lost and shoved it up in the loft with the other junk.

Fast forward to late last year and I found it again and decided to have a mess around with it. Not really understanding the theory of these antennas, I tested it for continuity and was a bit shocked to find exactly what you found, that all the elements were grounded and that the centre of the coax connection went nowhere. I then did some reading up on the subject and things became a little clearer, but I have to confess that if I was put on the spot and asked to explain how they work, then I'd still struggle.

I wound some threaded brass rod into the main body of the boom part and threaded the broken element back onto this as a repair, sealing it with Araldite as the others seem to have been - it's the longest lower one on the left, as seen in the picture below - I think it's originally been home-brewed from old TV aerial bits.

The way that the 'U' bracket was originally fitted it looked like it had probably been last used 'horizontally' polarised, but I was going to use it 'vertically', so repositioned the bracket. However, before this, I stood in the living room holding it by hand and pointing it around to receive various signals and I got the distinct impression that it worked better when it was as I perceived as being upside down, that is, with the Gamma match part facing downwards. I tried doing some research and couldn't find any information on this, mainly because most were being used horizontally and I couldn't seem to find any written theory on this at the time. I found only one picture of a similar design where someone had got it jammed, I think, between books on a book shelf in a room and it had the Gamma facing down, so a bit inconclusive. Now it could just be some sort of ground plane effect with me holding it just at head height in a downstairs room and in the real truth is that the aerial can be operated either way up with exactly the same results when at height - or can it?

Just for a bit of fun and experimentation I put it up on a thirty foot pole with other antennas and decided to go against what I'd found at ground level and mounted it the way I would have thought it should be with the element facing upwards. It actually worked really well, other than with it not being on any sort of rotator. I had to stand at the foot of the pole with a hand-held connected via a long length of coax going into the house and rotate the pole by hand until I homed in on a particular transmission.

This was last year and it's all been taken down now. It was just a bit of fun and experimentation and it certainly worked better than the 'omni' when beamed onto a particular station, but would it have worked better the other way up? It was just too much trouble putting the pole up and down to try it different ways up and hoping that the same stations and conditions affecting them would happen to remain the same between these operations.

I can confirm that the shorting bar which also acts as a stabiliser for the element and made to be movable on mine will alter the SWR. Pictures below - someone will probably recognise that antenna and say "I made that years ago, how did you get hold of it"

[SiriusHardware]

When using the one which started this thread, handheld as it is designed to be, I didn't find any noticeable difference whether I held it matching element up or down. The fact that all the elements are shorted to the boom is, as you say, disconcerting. This one has a shaped resin (insulated) handle which is really only comfortable to use when the aerial is used horizontally so when using it vertically it is more comfortable to hold it by the back end of the boom, and I did wonder if I was compromising the operation of the aerial by doing that.

A more embarrassing story, which I may have told before, concerns the first CB radio base aerial I ever bought, which was a full size quarter wave groundplane with three elements, spaced out at 120 degrees and pointing straight out horizontally from the base. It came with an odd looking flat loop of metal which, if fitted as per the instructions, would have shorted the bottom end of the vertical element to the ground plane.

I took one look at it and thought - "I'm not fitting that, that's going to short out my transmitter output!". It wasn't until many years later that I realised it was a matching device.

[Terry_VK5TM]

Originally Posted by SiriusHardware

Do I take it that the sleeved / insulated input connection is really just a poor man's variable capacitor, the value of which is determined by the position of the shorting bar?

I meant to answer this in my other post - that is exactly what it is - a variable capacitor and not so much "poor man's" either - think piston trimmers etc.

[SiriusHardware]

I used that phrase in the sense that a short length of coax inside aluminium tube is likely cheaper (and probably less physically delicate) than a decently voltage rated trimmer capacitor.

I've seen versions of the gamma match which actually use the outer braid of the coax as a substitute for the short aluminium element - disconnected by an all-round cut at the feed end, and bared at the other end so that the shorting bar can connect it to the nearby main element.

[Terry_VK5TM]

Yeah, I should have put a smiley on that bit

Ive got a 3 element yagi that uses an actual capacitor in conjunction with the gamma match, not weatherproofed as I only used this one hand held on a fine day

Might be a bit small to see properly, but that little orangey blob is a ceramic cap.