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Hairpin matching

W5LZ said:
QRN,
This is being 'picky', but might clear up some misconceptions.
From "Astronwireless"s quote, power is being fed to both sides of the driven element. One side isn't directly connected, but through a capacitor/capacitive network. It still gets to that 'other' side.
The amount of skew of the radiation pattern is not a major thingy unless you are direction-finding. It's also not a 'fixed' amount, varys from antenna to antenna and environment. (Some environments, installations, introduce some skew all by them selves.) In most cases the amount of skew just isn't very critical at all, and can be compensated for if your 'numbers' just have to be 'dead nuts' accurate. Until you get into the VHF/UHF regions that skew is a very small portion of a directional antenna's beam width. At HF, it's probably less than the accuracy margin of the rotor control's indicator. (If it ain't, you'd better find another compass - lol.)
For typical HF use a gamma match isn't a bad deal at all. Sure it has 'qwerks' you should be aware of, but in the majority of cases, that 'awareness' is all that's required. If you wanna change from a gamma to a beta match, be my guest! If you don't want to, no biggy with that either. If you're into microwaves, radar, and such, dump the gamma match. Or, do some calibrating.
- 'Doc
Click to expand...

Yeah Doc I did not care for how they said that power is only delivered to one side of the dipole element. That is a poor way of describing it I will admit.It is simply off center feed.Again no arguments from me about how much skewing will occur at HF. As I said the problem becomes more pronounced the higher you go.That being said I did notice quite a differance on 6m. Folks about 1/2 mile up the road had a baby monitor on 49.890 MHz that made a dandy signal source and since I could see their house from the top of the tower I knew the pattern was off by sighting along the boom.With the hairpin match it was dead on.In no way do I think the gamma match is garbage it's just that I personally had better luck tuning with a hairpin match and saw real world improvements in pattern.As for any differance in gain,I can't see where that would even be measurable except on an antenna test range.
 
try referencing Cebik and gamma match properties at HF.

http://www.cebik.com/trans/gamma.html

One criticism of the gamma match accuses it of displacing the pattern in the direction of the match due to the size of the assembly. Effectively, as some believe, the assembly increases the diameter of the gamma side of the element, and this asymmetry of the driven element results in the main lobe's re-aiming. To test this notion, I constructed models of 28-MHz beams with identical reflectors and element spacing. One beam uses a simple driven element. The other uses a driver with the same overall length, but with a gamma match assembly (of course, with no boom). The gamma assembly is in the plane of the two elements and projects forward of the driver. The results of the test appear in Fig. 2.

gamma-2.gif


(image hosted by FireCommunications Network)

The slightly lower forward gain of the gamma-matched version of the beam is an artifact of certain limitations of NEC (which we shall review shortly). The key factor in the overlaid patterns is the degree of lobe displacement, indicated by the lines that I added to the sides of the lobe. Displacement does indeed occur, but at a level too small for any user ever to notice in operation. The two patterns to the right show that the gamma match also has an affect on the free-space side nulls for the array. The simple beam has side nulls that show no limit. However, the gamma-matched beam has limited side nulls that are a mere 40-dB down from the level of maximum gain. I am unaware of any operational use of a beam in which one might be able to detect the difference.

The modeled test case assumes by its mathematical basis relatively perfect construction of the gamma-match driven element. I have in past years seen range-generated patterns for gamma-matched beams with a significant displacement of the main forward lobe. It would not be possible to perform a full analysis of such patterns without being able to model currents within the driven element and along the boom--if the elements makes a direct connection to the boom. However, in principle and assuming careful construction, pattern displacement is not a hindrance to the use of a gamma match.

http://www.cebik.com/trans/gamma2.html
 
No where did I or anyone else that I can see said that any pattern distortion was a hinderance to using a gamma match. What I did say is that I saw a definate differance in the ease of obtaining a perfect match and that I did in fact see a better pattern that was not skewed. Every installation is differant and everyone may get differant results. I know what I saw and I know enough to determine what I saw was real and not wishfull thinking.Use whatever suits you.I won't try and pick apart your decision or reasoning unlike what some others will do. :roll:
 
there is no pattern distortion or "skewing" as long as the gamma match is constructed and configured correctly.
 
A7DX said:
there is no pattern distortion or "skewing" as long as the gamma match is constructed and configured correctly.
Click to expand...

Yes,we heard you the first time.You have your, as yet unfounded, opinion and then there are posts all over the net and in print that claim the opposite.I was asked for referances to that and they were provided.I won't argue about HF but VHF and UHF are another thing. Like I said, use whatever method floats your boat.I don't care.I happen to find a hairpin easier to construct and tune and it is more easily reproduced when building several of the same type antenna.Also try and gamma feed a triband yagi and see what happens.Let's just agree to disagree and move on with more important matters in life like when will 10m and better yet 6m start opening regularily? ;)
 
I always love it when someone provides a link to a site that you must be logged into in order to see the info. :roll: After looking around there it appears to be some site that freecell, or 228, is associated with.
 
I'd sure like to see a better picture or illustration of the match with measurements so I can make one for the Maco M104C I'm building. I've looked all over and can't seem to find anything.
 

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