Ralf,
Thanks for catching my mistake in the ohm's law formula.
P = E^2/R
Regarding using the far-field loss formula it is probably prudent to
try measuring power on the receiving antenna when transmitting on the
other antenna (the yagi). At the very least check with a SWR meter
in the lowest power range and see if you detect anything. If the
meter deflects or "twitches" power is probably way too high and you
need some kind of protection device. There are some simple milliwatt
power meter designs in some ham Handbooks (look for field strength
meters); simplest is a IN34 and 1ma meter. If you blow up the 1N34
you have your answer! Better that you use a couple 20-dB coax
attenuators before the meter at first. I no power is seen then
remove one and test again.
You can rely on using 0 dBm as maximum survivable input to the
receiver, but the receiver still will be driven into compression and
not usable while transmitting.
My example of 130-feet was at 144-MHz so not a fair comparison with
HF freq. which have much longer wavelength.
The space loss formula is useful for making measurements at far-field
(google it)>
73, Ed - KL7UW
----------------------------
From: Ralf Wilhelm <r...@super-deutschland.net>
Cc: "Elecraft@mailman.qth.net" <Elecraft@mailman.qth.net>
Subject: Re: [Elecraft] Antenna question
Message-ID:
<8dbfe76b-9bb4-4da1-a0b9-4a507d1ae...@super-deutschland.net>
Content-Type: text/plain; charset=us-ascii
Hi George,
The 130 feet corresponds to lambda/2 on 80 and you can use the far
field approximation (that Ed is using) there...
At short distances (less than a quarter or a sixth of the
wavelength), however, 1/r^2 and 1/r^3 ("near field") components of E
and H fields are still present (or dominant) and the far field
approximation should not be used. You also have to be careful with
the cross-polarization argument, since the electrical near field has
all three vector components almost anywhere in space and the coupling
can be much higher (depending on how well symmetry is preserved in
the "yagi+vertical system").
Better use a NEC based program (e.g. EZNEC or the free 4nec2), if you
have on access to a milliwatt-meter/scope and have to calculate...
By the way, P should read
P= E*I = E^2/R
=> E=sqrt(P*R)=223mV
for a 0dBm (S9+67dB) signal (?)
Greetings
Ralf, DL6OAP
73, Ed - KL7UW
http://www.kl7uw.com
"Kits made by KL7UW"
Dubus Mag business:
dubus...@gmail.com
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