This is a reposting. I sent my original in HTML. Apparently that is a no
no.
So here is PLAIN TEXT!
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Since I cannot get open wire line into my shack I am considering using a run
of low loss cable, 7/8 heliax for example (because I have some!) about 100
feet to a balun at the tower. The balun would be mounted up perhaps 10
feet from the base of the tower and I would run open wire, such as the 450
ohm 14 ga full legal limit ++ from Cable X-perts from that point up to the
antenna feed point at the 80 foot level. I want to use the antenna on
160/80/40.
Its going to have to be an inverted-V, with the ends connected to sturdy
trees up about 30 feet. These trees are available 190 feet from the tower
on each side, so I could make the overall length of the inverted-v up to 375
feet long.
I have 2 questions, which are likely dependent upon each other:
1. Should I use a 1:1, a 2:1, a 4:1, a 6:1, a 9:1 a 12:1 balun? All of
these are available from Amadon, and they claim they will handle full power.
I wish I could verify that before I blow one up at full power and 10:1 swr
however!
2. How long should it be. The impedance it presents on each band will be
highly dependent upon length. The ARRL Antenna handbook has graphs of
impedance (resistive and reactive) for an example dipole of 100 feet. For
typical wire this resonates at about 5 Mhz where the impedance is about 75
ohms. At 10 Mhz the impedance as I recall was very high, over 1000 ohms,
this is the case where the center is a voltage maximum. As the frequency
increases the curve wraps back, crossing zero reactance, then back again
etc., it eventually spirals in at 377 ohms resistive. I have scaled that
example down in frequency to suit my question. A 375 foot long dipole would
resonate at about 1.2 Mhz. It looks to me like it could be a compromise
match at 160 and 80. If I cut it to resonate at 160 I believe it would be a
bear at 80. I want to pick a length which allows me to reasonably match the
antenna on 160, 80 and 40. I will be using a Nye Viking tuner.
Here is my guess: Get a 1:1 balun (voltage balun, essentially a 1:1
transformer on a ferrite core) and let the tuner deal with it. Then by trial
and error trim the length of the antenna so that I can readily load it on
160/80/40. BUT maybe those of you with real experience can answer this
straight away. EZNEC might help too.
Ron N5IN
Addendum: Actually since I know from the curves that the impedance is
likely to range from about 75 ohms resistive to perhaps as much as 1000 ohms
(I am trying to avoid that extreem case by comming up with just the right
length), and I know that the impedance zeros in on 377 ohms, perhaps a
better choice would be 377/75
= about 5, thus maybe a 4:1 balun would be a better match.
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