Greetings inverted-L fans,
In regards to the 'proper' capacitor to use at the feedpoint
of a 'full-length' (say, 170' long overall) Inverted-L antenna:
Thank you for, and here is, the wisdom of the reflectorites:
Much appreciated,
73, Glenn, KB1GW
---------------------------------------------------------
My original posting:
>Okay, so I'm thinking about extending my 1/4-wavelength "inverted-L"
>out to 170 feet (+/-) and putting a (air-variable) capacitor at the
feedpoint.
>I know I'll need something like a 500 to 1000 pF cap. However, [at] what
>voltage rating? And, if you have a brand/catalog number, that'd be great.
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Replies received--sans names and call signs:
"Since the cap is in series with a "current" point on the antenna. HV
ratting is not as important as current ratting. a 2KV cap should be
plenty. In fact If you can build a suitable enclosure then even Air
variables work quite well. I use a 150pf air variable shunted with
several of the old surplus 100pf transmitting micas (the brown ones abt
2" square). For example to 500pf TV style doorknobs wont work because
they cant handle the current (they get warm even with 100 watts) even
though they are rated 15kvto 20kv. At 1KW the current can be on the
order of almost 6 amps (more on peaks). If you plan on using the
antenna on other bands (say with a tuner) then be careful you can end up
at "voltage" node on another band and blow the caps!I that case the
vacuum variable is more likely a better bet. I have had the best luck
finding vacuum variables at HAM fests I have paid anywhere from $35 to
$80 for them.
PS: I have two L's in this configuration--fed in phase... good luck"
=====================================================
"My inverted L is abt 160' long (60' vertical). I used fixed+variable
caps to achieve a reasonable match. My fixed was a 500pf 5kv old bathtub
cap and I used a section of RG213 in parallel with it as my "variable" (you
can
trim the length to achieve best match). My bathtub went bad (heated/leaked)
so I was left with abt 12' of RG213 alone (abt 20+ pf/ft). The match is now
abt
1.2:1 at 1840 and rises to 2.1:1 at 1800. 2:1 SWR BW is just under 80kHz,
which seems to indicate a reasonable ground (I have abt 1500'of many
short radials) and it seems to play!! I think RG213 is good for a few KV
and
is a lot cheaper than an air variable. Just a thought. 73 es Happy New
Year"
=====================================================
"Something up to about 1000 V should do the trick. This is the same sort of
cap one would use as the output cap in a linear amplifier's pi-network.
Remember: P = E*E/R. If the series resistive part of the antenna's Zant is
approx 50 ohms, then for 1500 W, E = SQRT(1500*50) = 372.8 V rms = 387 V
peak. So, a 1000-V cap should do the trick. 73"
=====================================================
"I'm getting ready to put up an inverted-L here, and I'm thinking of
using a length of coax for the capacitor. It would be pretty long, about
30' ( ~30pf / ft for most coaxes) but cheap and no problem with the voltage
rating (2000 - 4000v, depending on the coax type). In the ARRL Handbook,
they say you need a 1500V variable cap for the 175' inv-L. Not easy to find,
or inexpensive. 73" [See posting above. --KB1GW]
======================================================
"Glenn, I have an inverted L like you are shooting for (maybe a bit longer)
I use have used some old caps with large spacing but am currently using
the old Heathkit remotely tuned cap. It has rather small spacing but doesn't
arc even at 1400 watts. I am also using an inductor so I really have an L
network.
I have also used coax as a cap. I would imagine some RG-8 would work with
about 5000v rating. Old 5 kv ceramics can be used as 'padders' if you can
not
find a large value cap. Actually, I only use about 250 pf to tune mine.
Good luck and CU on topband. 73"
=======================================================
How about a roll of coax to tune out the reactance of the inverted L.
I have used pieces of coax to tune antennas for years. The voltage
breakdown is high and it stands up to the environment better than bread
slicers, and is a good bit cheaper. Just trim it to the required length. I
cut the
braid back on the end to increase the voltage breakdown. Then coat it Q
dope and/or electrical tape. RG 8/58 is about 30 pf per foot. You can tune
up with a variable and then replace it with a cable cut by either measuring
the
variable or estimating the Cap. If you want to tune different parts of the
band, consider switching in or out different or additional pieces of coax.
I had a T antenna on 80 that was tuned with a 5 ft length of RG 8 taped to
the
bottom end and running back up the antenna. The shield connected to the
antenna and the center conductor connected to the coax. Hope this has
helped give you a few ideas. Good luck on topband. 73
[See "Table 1," page 24-17 in the ARRL Antenna Book (17th edition)
for characteristics of commonly used transmission lines. --KB1GW]
-(feedline ctr conductor)->(-coax 'x' pF per ft._)_shield of coax cap to
ant_
==============================================================
"Remember that adding length to the inverted L will help, but most
important is that the vertical portion be as long as possible. All you'll be
doing by adding length to the horizontal portion is making a bigger cap.
hat. Good luck"
[And,]
"I've only been playing with verticals for a short while but have reading
lots of material, therefore, my suggestions are unfortunately not from
experience but theoretical.
An inverted L is really a short vertical with a capacitance hat. The hat
does two things. First, its size will dictate the reactance component at
the feedpoint. Second, it will improve antenna efficiency. By adding to
the horiz. length [of the 1/4-wavelength antenna], you'll be changing the
reactance (hence, necessitating a change in your current LC configuration)
but will slightly improve the antennas efficiency. If you can get the
vertical
portion longer you'll then be improving the "low angle" characteristics.
If you got the time, experiment with many options. Would love to hear
your results. I'll look for you during CQWW at months end. I currently
have a center loaded, 40 foot, vertical that seems to play OK, but it was
my first 160 vertical, so I don't have alot to compare to yet. 73"
=========================================================
>All you'll be doing by adding length to the horiz. portion is making a
bigger cap.<
Not completely true - by adding horizontal length *beyond a quarter-wave*,
the current maximum is moved "up" the vertical section. If the wire was
one-half wavelength long, for example, the current maximum would be at the
wire's midpoint. If the L is exactly one-quarter wavelength long, the
current point is at ground level. Raising the current maximum away from
the ground should reduce ground losses and improve low-angle radiation.
How far the current maximum should be raised depends on how long the
vertical section is. I would model the L with different horizontal
lengths and see how the pattern changes - expecting that adding length
would first improve low-angle radiation and then start to fill in the
high-angle pattern as more current begins to flow in the horizontal
section. I would also expect that there would be a fairly large range of
vertical/horizontal ratios for which the pattern changes very little. 73
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