gamma match capacitors

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Joined: Wed Mar 03, 2010 2:39 pm

gamma match capacitors

Post by K9HY » Mon Jun 13, 2016 2:15 am

I have and five-band Cubex long boom quad I want to put gamma matching
caps and also :?: :?: tuning caps...

From "ALL" the reading I've read from old W6SAI's and the ARRL Handbook NOTE everybody has something different in their cap values.* None tell of what voltage values
or EVEN IMPORTANTLY where they got these caps at. Some say 50 pf's for the tuning like the Directors and the reflectors. The main ones the driven elements are vague and their cap ranges in different values. Local electronic supplies are a nil here. Radio Shack is actually and telephone/computer shack now...And they're far from being a good quality electronic parts supplier... Heck1 They didn't even have the alligator clips I needed for my analyzer to hook up down the road. Had big ones not the small ones...

I have 1/4 y matching coax 75 ohm to the 50 ohm's...

Biggest concern> I have and linear so I'm very concerned and of course scared to screw up my linear... I have a nice tuner but...BUT i get tired of having to re tune on each band
change... My antenna analyzer shows way different center freq's than desired... I want to do this right/correct...

Like some help on this subject.

Tnx 73's

Steve K9HY

Posts: 18
Joined: Thu Oct 20, 2016 10:52 pm

Re: gamma match capacitors

Post by KC6NEN » Thu Feb 16, 2017 6:04 pm

I build Tesla coils, and I've found I need to give the caps 3x the voltage rating for the resonant rise of the coil. Inductors; 3x the current in the circuit.
So, if you assume the point is at 50 ohms; if you are running full 1500 watts (not allowing for modulation, power in side bands): this is : P=E 2/R, or 1500= E 2/50, or: 1500 x 50 = 75,000 = E 2; so; square root of 75,000 is E , or: 273 volts. Times 3 for a safety factor is 821 volts, or 850v, 900v, 1 Kv. Round-up to whatever is closest made value.
Off resonance, the voltage may rise out of phase with current--maybe even more than this.
Sometimes polystyrene caps work on HF. Vacuum caps , ceramics, too.The old silver mica xmitting caps were great, available surplus. Or those big Doorknobs.
Any cap needs be rated for the current. The wire leads must be good for 3x the anticipated current in the circuit, per Ohm's Law; P= I 2 R. A #24 wire is good for about 1 amp, so if you anticipate 3 amps, it would need about #13 wire leads.

Hope this helps

Posts: 2
Joined: Wed Mar 03, 2010 2:39 pm

Re: gamma match capacitors

Post by K9HY » Fri Jun 30, 2017 9:42 pm

My apology Ken for a late, late mni. mni. tks and your cordial input. I very rarely make it on this site but
have taken notes to your tech. help which is a great help.

I have not had much help in finding the correct variable caps yet and I am utterly amazed Cubex or some genius Ham
has not made kits or pre-made kits for quads. Really because many Hams do love to use these worldwide and yet nobody
makes these necessity's out there for these to use to easily tune. Heck ! These old verticals and the yagi's are galore
with such things. Sky is the limit ! I am amazed how much a vertical antenna is and gadgets and tuners for these.
Unbelievable and personally I think it is crazy and gotten way out of logic where I see major stores sellers using magic
to newbies---DX! Work DX!

My getting my 5- band Cubex antenna getting a major over haul was put on back burner here til recently and I have that itch I can not rid to get it done. I have been brainstorming on many little things on how I'm going to do this and that. One thing though has been bugging me is the hairpin method used for beams <?> That actually is and older but quite easier and reliable deal vs. money for expensive baluns and high priced
tuners as big as a KW linear like Palstar and others... Hi! I have and old US Air force beginners training manual on radio antennas and in it has a two-band beam with separate driven's like 10m and 40m like the capital letter" H" if your looking from way over-head down on it or at and 45' angle from below ! Nobody does that except KLM you know was using a Rohn tower as supporting boom for huge 40 or 80m HF Beams. And Hi-gain had the same techniques of multi-band antennas but I don't know if they market those anymore like they use to<?> But they didn't use the hairpin. I just am brainstorming if I can use that using a sturdy support for my quad and avoiding using outdoor Gamma caps altogether or not <?>
My old elemers are all SK now..I'm 57 here...

Again mni tnx and again I do apologize for not cordially RSV'ing back as I should of. I actually just discovered your RSV Tech feedback a month ago and have been busy with my family and work.

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