[Elecraft] (no subject)

[Edward R Cole]

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 <ralf at super-deutschland.net>
Cc: "Elecraft at mailman.qth.net" <Elecraft at mailman.qth.net>
Subject: Re: [Elecraft] Antenna question
Message-ID:
         <8DBFE76B-9BB4-4DA1-A0B9-4A507D1AE0DA at 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
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