This past weekend the tower crew put up a 180 grounded tower at the
local 970Khz AM site and I have been asked to match it with a tuning box
which had been feeding for years an inverted L about a quarter wave
long. The 180 foot tower has three top loading guy wires each about 50
feet long with opes to get the three wire cage closer to 50 ohms. The
cage taps to the tower at about 50' and is adjustable. I was wondering
if someone with EZNEC could model this for me so I can get a head start
of getting it to take power. The ammeter is useless because even at
greatly reduced power of 100 watts the Nautel trips with a VSWR
overload. The station does not have access to a good impedance bridge
so out of frustration I ordered from DX-Engineering by overnight a MFJ
antenna analyzer that covers down to 530 KHz. The problem is that I will
most likely see an impedance and inductive reactance on the bridge for
which I need to make the ATU accept. The ATU is a bridge Tee design
but unfortunately it is set up with a coil on the input and one on the
output with the center of these two coils going to ground via a tapped
inductor and and then a 22 amp 2000pf capacitor going to actual ground.
I believe the original set up would and did handle both the "L" s low
impedance as well as take care of the capacitive reactance.But I do not
have a vacuum variable to deal with the expected inductance of the cage.
I am used to feeding cage fed towers with a C input, a tapped coil to
ground, and a C on the output to the cage. I have just the opposite. I
do have some fixed large current and voltage micas G2's and G3's But
two of them are 100pf a piece and one is 700 pf. I know of the trick of
put a flat wound coil in series with a capacitor to make it appear
variable and will try this based on the values the MFJ reveals. But to
save time I am putting out this request for someone with the time to do
a model with their EZNEC to save me time over the weekend when I should
be chasing DX.☺
Please send me your suggestions directly to avoid any congestion against
any of the current run of exciting issues.
Thanks,
Herb Schoenbohm, KV4FZ
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