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Old 14th Apr 2020, 8:55 pm   #1
G6Tanuki
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Default "Gamma match" calculations.

To relieve the boredom of lockdown, I've been playing with antennas.

My current project being a 14MHz 1/4-wave built from a 16-foot, 2-inch-diameter galvanised scaffold-pole with a little 'sprig' on top to make it 16-foot-6-inches.

The bottom is fitted to a galvanized 'fence-post-spike' as ground, and provided with four 1/4-wave radials slit-cut into the lawn.

I'm experimentally feeding it with a 'Gamma match' using a 2-inch exhaust-clamp to connect to the mast near the base (it's easily slide-up-and-downable) with a length of 8mm rod attached so it sticks out at 90 degrees froim the mast, then a 16-gauge copper wire from the rod, running down parallel to the mast to the feed-point.

I know I could do it by 'hope and grope' methods to determine the correct length of the Gamma-element and the correct spacing of the gamma-element from the main mast, but does anyone have proper equations so I can do it more scientifically?

I've seen some simplistic ones but they all seem to assume the Gamma-element and the radiating-element are of the same diameter, which is definitely not what I have!
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Old 14th Apr 2020, 9:52 pm   #2
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Default Re: "Gamma match" calculations.

MMANA-GAL or other free antenna simulators are worth a try, you can lose days playing aerials. My mate has tried the Whisky diet, lost two days last week.
 
Old 15th Apr 2020, 6:05 pm   #3
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Default Re: "Gamma match" calculations.

That's not one I've come across before: thanks for the heads-up!

I'm happy to do 'grope and hope' experimentation on these sorts of things but when I get something that 'seems to work' I'm always enthusiastic to see if the practice matches the theory (or the other way round: these things should be symmetrical!) and then see if I can't use the combination of theory and practice to make it work better.
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Old 18th Apr 2020, 11:47 am   #4
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Default Re: "Gamma match" calculations.

If your vertical was a half wave you would gamma match feed it.
As a quarter wave couldn't you make it a folded ground plane - ie the feeding wire would run all the way to the top?

MMANA-GAL has one of these in \HF Simple\Vertical\Folded GP_50Ohm.maa
(It has the gamma matched half wave too which is fed at about 1/8wave up from base)

Last edited by Jon_G4MDC; 18th Apr 2020 at 11:58 am.
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Old 18th Apr 2020, 5:30 pm   #5
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Default Re: "Gamma match" calculations.

Quote:
Originally Posted by Jon_G4MDC View Post
If your vertical was a half wave you would gamma match feed it.
As a quarter wave couldn't you make it a folded ground plane - ie the feeding wire would run all the way to the top?

MMANA-GAL has one of these in \HF Simple\Vertical\Folded GP_50Ohm.maa
(It has the gamma matched half wave too which is fed at about 1/8wave up from base)
Thanks for that! I hadn't thought about making it a folded-1/4-wave! Doing so would make the earth-resistance [I already have a reasonable earth-mat based on lengths of 2.5mm T&E spade-slotted into the lawn] but the soil here is shallow with a chalk underbase and so anything that makes 'ground losses' lower will be of value.
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Old 19th Apr 2020, 10:21 am   #6
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Default Re: "Gamma match" calculations.

No problem! If you haven't tried MMANA-GAL it is well worth downloading.

I hadn't put much thought into gamma matches before and it was a chance to play around with that and with the effects of ground conductivity and height of base+radials above ground. Nice to visualise it.
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Old 19th Apr 2020, 1:11 pm   #7
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Default Re: "Gamma match" calculations.

A warning...
Simulators are fun, you can lose days playing. Maybe not a bad thing considering the current situation*.

*Corona Virus pandemic lockdown for historic readers of this thread.
 
Old 19th Apr 2020, 1:57 pm   #8
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Default Re: "Gamma match" calculations.

Have a browse of any amateur radio books you have (or google) that feature quad or yagi beams for 20m, any that show gamma matching you can copy and will get you there.

You could use a VNA and measure the admittance and apply that to the Smith chart. WINsmith is very good (and free) calculator and will give you various configurations of L and C matching to get to Z=50 ohms resistive.

Quicker and more fun to "cut and try" especially as the wx is nice
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Old 20th Apr 2020, 6:16 pm   #9
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Default Re: "Gamma match" calculations.

I've been playing today on my 'antenna range' [otherwise known as the orchard].

The 'folded 1/4-wave' or 'gamma-match going all the way to the top' does seem to work - but I've also been experimenting with what happens when you progressively move the base of the 'gamma' away from being parallel with the main vertical and make things more into a short-based triangle.

Sort-of a 1/4-wave vertical fed by a 3/8 wave 'sloper' - so 5/8 wave in total. A series-inductor (roller-coaster) between the bottom of the sloper and the TX seems to match things nicely.

Haven't really pondered the radiation-pattern but I'm thinking that the main vertical-element will give low-angle radiation and the radiation from the sloper wire will be at a higher angle, which could give me the best of both worlds !

In these 'lockdown times' I've got plenty of time to play with antennas in the orchard.
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Old 21st Apr 2020, 10:14 am   #10
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Default Re: "Gamma match" calculations.

If you can, keep an eye on a field strength meter placed as far away as you can see it (use binocs if necessary) while you try out these various forms of matching.
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Old 21st Apr 2020, 10:23 am   #11
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Default Re: "Gamma match" calculations.

Yes, I'm using a field-strength meter located about 1/4 mile away!

I view its meter by making a video-call to an old phone that's cable-tied to a branch a few inches from the FSM. So much easier than using binoculars/telescopes.

[And yes I've tested the FSM to make sure it's not getting misled by the local RF from the phone]
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