Re: [TowerTalk] 1/8 wave spaced 80m verticals

[Jim Lux <jimlux@earthlink.net>]

At 03:50 PM 9/10/2003 -0400, n4gi@tampabay.rr.com wrote:

> > At 01:07 PM 9/10/2003 -0400, Roger D Johnson wrote:
> > I toyed with the idea of feeding
> > >the elements in phase for transmit and with the delay for receive.
> > >It made the system a bit more complicated but resulted in better
> > >transmit signal while still having the F/B on receive.
>
> Exuse my unscientific anecdotal questions, but does this mean if I didn't 
> care about F/B (I have RX loops for that), only TX gain, I could just feed 
> my 1/8WL spaced verticals in phase?
>
> Is TX optimization any less complex to accomplish than the F/B RX 
> scenario: (i.e. no network analyzers/calculus spreadsheets needed)?

Well... if you're feeding them in phase, it's pretty simple... no 
calculations needed because the mutual terms don't result in any current 
flow: think of the two antennas as a pi network, with the mutual impedance 
being the series "bridge" and the self impedances being the shunt legs of 
the pi.  If the voltage at the two ports of the network is identical, no 
current will flow in the series element, regardless of what it actually is.

If you feed them out of phase, you're basically building a W8JK type two 
element beam, where the gain increases as you bring the elements closer 
together, but so does the relative losses.   The actual current gets quite 
high too (think of that series element in the pi net... closer together 
means lower impedance, so more current into the feedpoint)


> Even if it isn't too complex to optimize 1/8WL spaced verticals for TX, 
> some have noted only a minimal TX gain (~3db) when phasing 
> verticals....  Is this the norm?

Short answer: it depends

The pattern you get will be sort of figure-8 shaped, with the narrowness of 
the lobes determined by the spacing of the antennas.  In the limit, with 
the two antennas side by side, the pattern is identical to the pattern of 
one element.  At 1/2 lambda apart, you get a null (ideally to zero) for 
signals coming from along the line connecting the two elements. The main 
lobe is 60 degrees wide (3dB down points)  Start spacing wider, and you get 
multiple nulls and lobes (you're basically building an interferometer) at 
3/4 wavelength, the main lobe is 40 degrees wide.





> Sounds like I might be better off taking one my verticals down and using 
> the metal for ground rods on the remaining one  :-)
>
> 73,
> Blake N4GI
>
> _______________________________________________
>
> See: http://www.mscomputer.com  for "Self Supporting Towers", "Wireless 
> Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with 
> any questions and ask for Sherman, W2FLA.
>
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See: http://www.mscomputer.com  for "Self Supporting Towers", "Wireless Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.

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