Tony,
At what distance did you compare the 2 antennas. If the doublet is a
"flat-top", what do you mean by its "apex"?
A 100 foot center fed dipole 30 feet above "average" ground on 20M will have
2 narrow lobes broadside and 2 wide lobes off the ends. The greatest nulls
are not off the ends.
"but there is one 30 degree swath of azimuth where the doublet is only an
S-unit or 2 below the Yagi." An S-unit or 2 could be 6 to 12 dB - that is
HUGE.
If DX is what you're after, you'll be far better off with a rotatable
5-element Yagi at 60 feet than a fixed 100 foot doublet at 30 feet. If close
in is what you want, you might have better results sometimes with the
doublet - depending on the direction.
73,
Marsh, KA5M
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of Tony
Sent: Wednesday, March 02, 2016 6:51 PM
To: towertalk@contesting.com
Subject: [TowerTalk] Doublet vs Yagi
All:
I have a flat-top doublet approximately 100 feet long with it's apex 30 feet
above the ground. It's fed with 300 ohm ladder line. I also have a
20 meter, 5 element mono-band Yagi that's twice as high at 60 feet above
ground.
When comparing the Dx reception of the two antennas, the signal strength on
the Yagi is many S-units above that of the doublet, but there is one
30 degree swath of azimuth where the doublet is only an S-unit or 2 below
the Yagi.
The direction where the doublet has the most gain centers about 20 degrees
off the wire dipole itself so it's far from broadside. It's as though the
gain exists close to where the greatest null should be; off the end of the
dipole.
My question is: what's most likely to cause the doublet to have that much
gain in one particular direction? Is it the terrain, the electrical /
physical properties of the doublet, the signals angle of arrival or some
interaction with the two antennas?
I should mentioned that the Yagi sits 40 feet behind the doublet in the
direction where the doublet shows the most gain. Someone suggested that the
ladder line may contribute some vertical component that helps reception.
Thanks,
73, Tony
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