[TowerTalk] 1/8 wave spaced 80m verticals
Jim Lux
jimlux at earthlink.net
Wed Sep 10 14:50:12 EDT 2003
At 03:50 PM 9/10/2003 -0400, n4gi at 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
>
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