To the first order, the feedpoint Z (at resonance) will depend on the height of
the vertical portion, which affects the radiation resistance. and the resistive
loss of the ground connection which appears in series with it. A full height
(1/4 wavelength) vertical over perfect (zero ohm) ground will be about 35 ohm.
A shortened vertical, toploaded (the "L" portion) will be lower than this, again
over perfect ground. Sixty ohm seems way too high and since you have a
respectable, but not outstanding radial system, suggests to me measurement
error. How are you measuring this?
My inverted L, 55 feet vertical, the rest horizontal measures ~24 ohm at
resonance, with a very marginal (work in progress) radial field of twenty, 55'
long insulated radials on the ground. I've used three different instruments,
all vector analyzers, to confirm this. (DG8SQQ, FA-VA5 and AA-55) I have a
50KW BC station on 1550 kHz that measures -3 dBm on this antenna. Only a vector
analyzer, used with care, will handle this.
Wes N7WS
On 11/18/2018 9:48 AM, Jamie WW3S wrote:
well, I THOUGHT I had a good ground.....16 radials I think, 1/4 long.....so I
thought I'd see around 30-35 ohms impedance.....not sure what to think
now.....I was going to get either a balun and unun at the feed point, was going
be someone else's statement that there inv l was around 22 ohms, glad I
measured mine before I ordered one....
----- Original Message -----
From: "F Z_Bruce" <k1fz@myfairpoint.net>
To: ww3s@zoominternet.net, "Topband" <topband@contesting.com>
Sent: Sunday, November 18, 2018 10:55:47 AM
Subject: Re: Topband: Impedance of inv l?
That sounds about right. As you put a good ground system under it, that value
will come down, and the efficiency will come up.
Many add extra antenna wire that pushes the current up the wire, this also
raises the impedance, hopefully to near 50 ohms with the right length.
A capacitor (variable, then fixed) in series at the feed point can cancel the
added inductance.
73
Bruce-k1fz
https://www.qsl.net/k1fz/beverage_antenna.html
On Sun, 18 Nov 2018 10:41:36 -0500, WW3S wrote:
What should the Z be for a 1/4 wave inv l, with the radials attached to a
radial plate? Mine seems to be 60 ohms or so....
Sent from my iPad
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