[Elecraft] Antenna question

[Ralf Wilhelm]

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


 

Am 12.02.2014 um 10:31 schrieb Edward R Cole <kl7uw at acsalaska.net>:

> George,
> 
> You do not say what power you are transmitting and it would be helpful to know the separation between the antennas. Will the vertical be as high as the yagi?  Need to know what frequency is used.
> 
> OK, in lieu of this data I will make some assumptions and show you the math to calculate the power received.
> F = 3.5-MHz
> Po = 100w = 50 dBm
> separation = 60 feet = 18.3m
> cross-pol loss = 20 dB (it will not be quite this much at this close separation)
> L = 32.4 + 20 Log F + 20 Log d, where F= MHz and d= km
> L = the space loss between antenna, in dB
> L = 32.4 +20 Log (3.5) + 20 Log (18.3/1000)
> L = 32.4 + 10.9 - 5
> L = 38 dB
> Ltot = L + Lcross-pol = 58 dB
> Po - Ltot = -8 dBm
> 
> That is a very strong signal but will not hurt the receiver.  Typically anything under 0 dBm is safe.  I run 1500w and have a preamp attached to the TR coax relay and have had no problems blowing up the very sensitive transistor in the preamp if the relay isolation is equal to the power in dBm.
> 
> Now cross-pol loss is probably not 20 dB but more like 15 dB and your antennas are in near-field so these formula are not quite accurate.  Ideally you would measure the power on the receiving antenna with a milliwatt power meter but I suppose you do not have one.  Also I did not include antenna gains in this, but in close proximity they are probably not accurate.
> 
> I have tested my tower mounted preamp by sending a test signal to an antenna about 130-foot away and using the space loss formula pretty accurately predicted received signal level.  This let me test the preamp sensitivity without removing it from the tower.
> 
> If you have a scope look at the received RF signal and measure the peak voltage.
> P = E^2*R or
> E = sq-rt (P/50)
> P = 0 dBm = 0.001w
> sq-rt(0.001/50) = .004v
> 
> 73, Ed - KL7UW
> PS: If you run more than 100w then either short the receiver antenna line or use a coax relay to disconnect it when transmitting.
> 
> ------------------------
> Date: Tue, 11 Feb 2014 23:36:09 +0000
> From: George Thornton <gthornton at thorntonmostullaw.com>
> To: "'elecraft @ mailman . qth . net'" <elecraft at mailman.qth.net>
> Subject: Re: [Elecraft] Antenna question
> Message-ID:
>        <040f2ec01a53458babf11914164d5177 at Server.thorntonmostullaw.com>
> Content-Type: text/plain; charset="us-ascii"
> 
> This might be a stupid question, but here goes.
> 
> I have a small lot.  I currently am using a 3 element Yagi that barely fits on the property.  I was thinking about getting a vertical as a second HF antenna.  If I put it up it is going to have to be pretty close to the Yagi.
> 
> I have a dual receive K3.  If I have both receivers going, one on each channel, and I broadcast on one of these antennas, am I in any danger of overloading and frying the other receiver?
> 
> 
> 73, Ed - KL7UW
> http://www.kl7uw.com
>    "Kits made by KL7UW"
> Dubus Mag business:
>    dubususa at gmail.com
> 
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