Real experience? Suggest you read antenna tuner article in March 1997
QST and my ad on page 154.
73 Paul N4XM
At 12:26 10/23/1997 -0500, you wrote:
>>>>
<excerpt>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 balan at the tower. The balan 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 balan? 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 balan (voltage balan, 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
</excerpt><<<<<<<<
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