> Anecdotal: In the 1980s, I had a 100-foot Rohn 25 shunt-fed
> tower with 200 radials. On top was a 204BA (4-el Hy Gain 20m
> yagi). The mast extended 12 feet above that. Later when I
> added a 4-el 15-meter Wilson yagi (17-foot boom) at 112 feet,
> there was no change in the shunt tuning needed or in the SWR.
Using EZNEC to do some quick model comparisons - looking for
trends and not exact values - the effect of a higher antenna
depends on the relative sizes and separation.
If the bottom antenna is much larger than the top antenna
and the separation between top and bottom antennas is less
than the turning radius of the bottom antenna, the top
antenna provides little or no additional top loading.
If the two antennas are of similar size and the separation
is comparable to or larger than the turning radius of the
bottom yagi, the upper antenna provides additional top
loading.
In all cases the current is divided among the sections of
tower/mast based on the relative size of the antennas so
the top antenna is less effective in combination than if
it were the only top loading. However, the electrical height
of the "system" is generally greater than the tower and top
antenna alone.
Some examples from Mike's case:
Both Yagis f = 2.27
Top Yagi Only f = 2.47
Bottom Yagi with mast f = 2.80
Bottom Yagi only f = 3.03
A similar set of models using an A50-5 (5 element six meter
yagi) six feet above a 204BA shows no significant change with
the upper antenna - similar to Jon's empirical results.
73,
... Joe, W4TV
> -----Original Message-----
> From: topband-bounces@contesting.com
> [mailto:topband-bounces@contesting.com] On Behalf Of Jon Zaimes AA1K
> Sent: Tuesday, November 10, 2009 8:37 AM
> To: Topband
> Subject: Re: Topband: Electrical height of short shunt fed tower?
>
>
> Anecdotal: In the 1980s, I had a 100-foot Rohn 25 shunt-fed
> tower with
> 200 radials. On top was a 204BA (4-el Hy Gain 20m yagi). The mast
> extended 12 feet above that. Later when I added a 4-el
> 15-meter Wilson
> yagi (17-foot boom) at 112 feet, there was no change in the
> shunt tuning
> needed or in the SWR. Perhaps if the higher antenna had been
> larger than
> the lower there would have been a difference.
_______________________________________________
160 meters is a serious band, it should be treated with respect. - TF4M
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