Gamma match and pattern skewing

[VK3LU]

I have heard of the above but has anyone actually had experience of it?
Is it noticeable on a 10 el 144 mHz yagi for example? I.e adifference between where the beam should be pointing and the actual beam heading.

Cheers
Nev

[VK2AAE]

From VERY rpt very limited experience I have seeen pattern skewed off boresight when
using other types of matching but the skew was UP not left or right.
Guess this doesn't help but .....

pat r
VK2AAE

[VK4WDM]

Hi Nev

I have a 10 el 2m Cushcraft yagi with a gamma match and an accurate rotator. When I am beaming from Townsville to the Atherton Tablelands the strongest signals in both directions are about 12 degrees further west than the expected heading. Now this could be the phenomenon you are talking about, but my strong suspicion is that this is simply a result of signals finding the right holes and/or reflections in the intervening terrain. There have not been any band openings since I put up the antenna to check other other directions.

73

Wayne VK4WDM

[VK3LU]

Thank Wayne
I suppose the only way to check it is over flat open country.
I did a search on this forum but found nothing. It's a curiosity thing mainly.

Cheers
Nev

[VK5ZLR]

My experience making 2m & 70cm gamma matched yagi's is that it doesn't skew off. Possibly if it was a really poor or faulty gamma match it might veer off.
But overall I think that if the match is basically OK, it tracks straight down the boom.

[VK3AUU]

The solution to the question is to do some actual measurement of field strength looking at a reasonably close source such as another yagi just into the far field. Sources such as beacons or distant stations are subject to reflections from all sorts of things, even local buildings. My instinct is that there might be some skewing with very short yagis but not with longer ones and it might be more noticeable with metal boom in contact with the driven element, but not with through boom insulated element.
Go to it you experimenters.
David

[VK4WDM]

Experiment! You want hams to experiment?

Wayne VK4WDM

[VK6ZFG]

Back in my younger days when active on foxhunts I made a direction finding setup with John (then VK6ZFD now VK6JCG) consisting of a windscreen motor driven rotating antenna on a roof rack and through the use of selsyns etc produced the antenna pattern on a long perstistance CRT inside the car in accordance with the direction from which the maximum signal was received.

The antenna used was a 4 element yagi (to Bill Orr design) but with a gamma match. Pattern skewing was evident and this was at the time attributed to the gamma match. I am pleased to see this subject raised as it fits in with our findings of many years ago.

Years later the drug squad tracked us down and asked for a demonstration. They were amazed at the speed and ease at which the fox was found. Their equipment at the time was rather primitive in comparison and not very effective. It just would not have been compeditive on a foxhunt!

[GM3SEK]

I have heard of the above but has anyone actually had experience of it?
Is it noticeable on a 10 el 144 mHz yagi for example? I.e adifference between where the beam should be pointing and the actual beam heading.

Cheers
Nev

I wouldn't use a gamma match with a VHF/UHF yagi. It works OK for impedance matching, but in modern times we need to think about more than that... things like RF interference, for example. We now know a lot more about the bad effects of unwanted common-mode currents on the outside of the coax shield and along the boom - and in all of these other important respects, the gamma match is a bad idea.

We know that the centre-fed driven element is symmetrical and 'naturally balanced'. But when we attach a gamma arm to one side of the centre point, that will actually create unbalance (which will typically be worse at higher frequencies). That in turn will cause common-mode current to flow on the outside of the feedline. Even worse, a gamma match will usually connect the centre of the driven element to the metal boom as well, so those unwanted common-mode currents will also flow along the boom and down the mast. This can be a potent source of RF interference on both transmit and receive.

That isn't just theory. Some years ago I built a gamma-matched 3-element Yagi for 6m - and the receiver noise level was S9! I knew this wasn't actually a "noisy QTH" and eventually came to realise that the problem was the gamma match itself. The mast and boom were acting as a large antenna collecting lots of local noise, and the gamma match was then injecting that noise into the coax feedline. The solution was to change to an insulated driven element fed through a common-mode choke, using a DK7ZB-style impedance transformer to handle the matching. The background noise disappeared almost completely and the weak-signal DX rolled in.

Since that day I have never used a gamma match again. Doubtless there are situations where interference might not be a problem, but life's too short to find out the hard way. There is always a better alternative that doesn't have those RFI risks.

Now returning to the original question about pattern skewing... this may be one area where the gamma match does not have a bad effect, or not much. Even though the feedpoint itself has become unbalanced, in a multi-element yagi I think you'll find that the parasitic elements tend to pull the pattern 'back into line' again. If someone wants to model that in more detail, feel free to go ahead However, that still isn't a good excuse for using a gamma match.


73 from Ian GM3SEK

[VK3LU]

Hi Ian
I have never heard of gamma matches introducing noise. I wonder if anyone else has experienced that.
Seems like the old gamma match is a can of worms.

Cheers
Nev

[VK3RZ]

Gamma match not recommended for any yagi. Quite apart from the unwanted noise pickup, which I too have experienced, there is a noticeable pattern skew. The skew might be barely noticable with a 3 element yagi, but with longer boom yagis, the skew can be enough that the furthest away directors are barely illuminated , killing off gain.
Bad business OM.
Mike

[VK3PY]

I have never experienced pattern skewing or adverse noise effects from 2m yagis with gamma matches. The usual 2m antenna for our field day operations (VK3UHF) is my old 12-el NBS yagi built about 30 years ago, employing a gamma match and elements in contact with the boom. We mount it on the same mast as our 26-el 70cm yagi (folded dipole feed) and 42-el 23cm yagi (folded dipole feed). We can usually hear the Mt. Gambier beacons on all three bands (we use them as a system "health check" on completion of setting up as they are 325km away). All three beam headings coincide. Ditto for the Mt. Anakie 2m & 70cm beacons which are line-of-sight about 20km away and all stations we work. Same goes for noise from the setting sun (another check we sometimes perform, especially when the sun is noisy, as it was during the recent Winter FD).

Very occasionally, for less-demanding operations, we take a 6-el 2m yagi with gamma match and a 15-el 70cm yagi with folded dipole, again both mounted on the same mast. Same results, too. No beam heading differences noted.

In the "olden days" when I first built and installed the abovementioned 12-el NBS yagi at home, and my local noise floor was essentially thermal, I could regularly HEAR (with ears, not computer), several big-gun 2m EME operators on moonrise. Guess where the moon was in relation to my beam heading? Case closed.

I think there is a lot of folklore surrounding this topic. VK3RZ above directly contradicts VK3AUU further up with an unsupported assertion which could be readily tested with modelling software. Who is right, and what evidence can each present?

Nevertheless, these days there are many proven yagi designs whose feed point impedances are such that they can be readily matched with a symmetrical, fixed feed arrangement such as a folded dipole. Why fiddle with a gamma match?

Chas
VK3PY

[VK3PY]

Re-reading VK6ZSG's posting, it occurs to me to question just how correspondents interpret the term "pattern skewing"? Are we referring to the antenna's forward lobe azimuth response being displaced from the boom's boresight (not my experience) or a pattern asymmetry in which the lobes either side of the boresight are not perfect "mirror images" (seen this often, especially with gamma matched short antennas)?

Chas
VK3PY

[VK3LU]

Thank you to everyone.
I threw this hand grenade because I have often looked at the gamma match on my 10 el 2m beam lying against the side of the garage (no it's not in use!) and wondered if the skewing theory was correct.
I am not entirely convinced that the gamma match is the best/easiest way to feed and as a result I am going to try a flat strip folded dipole one day.(one day )
As for 6 metres, I think the flat strip folded dipole would lack rigidity.

Cheers

Nev

[DJ2TX]

Hello everyone!
The old F9FT/Tonna beams for 2m+70cm had direct koax feed with only a 1/4 wave tube over the coax,grounded to the boom.
Well that didnt exactly worked as the balanced to unbalanced it was intended,lol
But Tonna was very honest about it,and the pattern you got with the instructions showed it.
The main lobe wasnt affected at all.But the 2 sidelobes next 2 it.One was about 2-3dB stronger than the other.Like one -15dB,while the other one -12dB or so.

That was it.

We used to run this antennas with a kW on eme in Europe and there was never a single problem i heard of.
As long as you had the coax taped very tight to the boom for some meters,that would create enough resistance on VHF+UHF to have no RF running down the braid.

I think these days one would use ferrit clips right an the feedpoint coax.If you choose the right type,that works on the hi frequencies.At lot of people do very well with that setup over here.

Best 73s,
Ollie DJ2TX

[DJ2TX]

But i must say one thing.Ian was right,when it comes to 6 meters.
I bought an old gamma matched 6m yagi.Recommended was to roll some coax,"ugly balun" style.
Well,talk about TVI like you have never seen.The neighbours hated me.Plus i had a lot more noise in RX,just like Ian.
It seems to me that at 6m and lower it is,at least,extremly hard to create any kind of balun that way.
While on 2m and up,even only taping the coax very close (for some meters) to the boom creates way enough resistance against HF on the braid...


best 73s from europe every one,
Ollie DJ2TX

[VK3LU]

Thanks Ollie,
You have convinced me! If I ever rebuild my yagis i will convert to balanced feed.

Cheers
Nev