[VHFcontesting] Re: Gain from an omnidirectional antenna

Dale Parfitt parinc at gte.net
Thu Apr 17 10:01:08 EDT 2003



Jim Worsham wrote:

> You are correct.  The horizontal loop gets its gain by not radiating very
> much energy vertically.  It's pattern looks kind of like a doughnut.  A
> horizontal loop will give you a few dB of gain but better than a yagi?  They
> have got to be kidding.  Sounds like you should be teaching them about
> antennas.
>
> 73
> Jim W4KXY
>

Hi Jim,
Modeling shows the classic shortened 1/2 wave loop (whether round or square) to
be about -0.5 dBi and have an eggshaped pattern about 2 to 3dB "out of omni". It
is important when measuring gain of this type antenna to  take   measurements at
different azimuth angles and average the results. A current mode balun is also
important to eliminate feed  line radiation.

Electrically (and mechanically) larger triangular antennas come in around
+0.3dBi
with a near perfect omni pattern. The slightly higher gain is due to the longer
electrical length. The patterns from these antennas can be made eggshaped
towards the front-back, or the sides, or nearly perfectly omni through element
length selection. Anechoic chamber testing confirms this.
See L.B. Cebiks site for a nice discussion on triangular and other omnis.-
http://www.cebik.com/ex6.html

I saw an interesting ad on Ebay not long ago for a homebrew square VHF omni that
claimed around 2dBd gain- as these are basically shortened dipoles bent so as to
fill in the nulls normally associated with a dipole- on has to ask- where is the
gain coming from?

So, as others have said, a single omni has less gain than a dipole's peak gain-
but does not need to be rotated as does a dipole in order to maintain that gain
figure.

Dale W4OP





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