Here is some info to get you pointed in the correct direction:
When you try to measure the telescoping tube type capacitance directly it
likely requires quite a bit of lead length to connect to the test equipment.
These leads are basically a high impedance transmission line with a length of
several degrees and produces an inductive value that has to be deducted from
the actual capacitive value. This correction makes the measured C much greater
than it really is.
To calculate the capacitance directly, see your ARRL Handbook early chapters
for the formula. It basically says Area of one plate (square inches) times
.224 times number of plates -1 (or 1 in the tubing case) = pF for one inch
spacing. You then divide this number by the actual spacing to reach a new value
for air dielectric. You then multiply this number by the square root of the
dielectric constant of the insulating material to derive the final pF value.
You in effect have two plates of differing areas due to difference in the
i.d. of the large tube and o.d. of the small tube. I would guess you can use an
average of these two values and be pretty close. If the insulator is a poly
material the dielectric constant will be in the range of 2.3 or so. You can
find good info on this with a Google search.
Using your numbers, the plate area average is 1.78 sq inches per lineal inch
and a capacitance of 6 pF per inch in air. Assuming a 2.3 dielectric
constant then a total of 9.1 pF per inch.
The tube capacitor can also be calculated as a transmission line problem
with more precision but this is a whole different procedure that is beyond the
scope of what you need since the device is adjustable in value.
In your case where you see a good match at 21400 KHz, I would be tempted to
increase the length of the driven element about 1 percent to see if the sweet
spot moves frequency closer to where you want it.
email@example.com_ (mailto:firstname.lastname@example.org) writes:
It's what I have and as pointed out, all elements are shorted to the
boom. I don't want to re-invent the wheel here, this is just a once a
year Field Day antenna.
That said, we have considered cutting the driven element in half
inserting some fiberglass, use a DX Engineering BEB-2 clamp, re-scale
the steps for proper length and test that. If it was going up for more
than 24 hours we would probably go that route.
For now, I just want to understand how a variable air capacitor is made
out of two sections of tubing in order to get me 85.5pF or provide a
range of say 50pF to 150pF.
I KNOW how to use coax to mechanically make a gamma. I KNOW how to
mount it. I know how to short it. What I DON'T know is how long to
make the tubes in order to provide me the desired amount of
capacitance. The best response so far was 20pF per foot. But per foot
of what? 3/4 tubing? 1/2" tubing? Where is the formula or something
concrete so I can...
1. Reverse engineer what I have so I can find out what I have.
2. Re-build what I have so it works, because right now it doesn't.
3. Learn how to build a tube air variable capacitor for future projects
and personal education.
It's not in the Antenna Handbook and Dave Leeson's book doesn't take
long for me to begin to glaze over. I've dug through Dave's book but I
see only where it makes reference to gammas. The actual formula must be
buried somewhere in there.
I'm willing to put in the effort and willing to RTFM, but I need some
help! Thanks and 73!
Dino - K6RIX
> -------- Original Message --------
> Subject: Re: [TowerTalk] The simple Gamma Match?
> From: K7LXC@aol.com
> Date: Thu, May 18, 2006 8:00 am
> To: email@example.com
> My question is why a gamma match in the first place? It's an unbalanced
> feed. My preference is a hairpin which is easier to build and balanced.
> Steve K7LXC
> TOWER TECH
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