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TopBand: Re: Flag & Pennants

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Subject: TopBand: Re: Flag & Pennants
From: (w8ji.tom)
Date: Tue, 15 Sep 1998 08:41:57 -0400
> Ron, K6UR asked:
> "Should I care about front to back ratio if I aim one at the Caribbean?
> There is nothing in the Pacific to cause me qrm so just wondering about
> if I should have concern over f/b ratio's?"
> The main purpose of a directional receiving antenna on 160m is to get
> better S/N ratio on receive.  The S/N will be better than a single
> vertical simply because of directivity.  Rejecting QRM from the reverse
> direction is of little importance unless perhaps you have a noise source
> you want to knock down.

Receiving antennas have different design goals from transmitting antennas.
The main goal of most transmitting antenna systems is gain, with a
receiving antenna it is directivity.

Front to back ratio is very important after dark, because noise propagates
in from all directions on 160 meters, in particular from equatorial
regions. That of course assumes your site is not limited by local noise. If
local noise is the problem, nulling the local noise becomes the priority.
Sometimes QRM is a priority, but most often it is noise.

Without QRM, S/N improvement is about equal to the directivity improvement
of the antenna. With dominant QRM or noise from a direction opposite the
signal, S/N improvement is equal to the f/b ratio. If the signal comes from
the direction of noise or QRM, you're just out of luck.

The EWE and Pennant are all phased arrays, the vertical sections acting
like verticals and the horizontal components acting like phasing lines. For
many types of noise even simple "two element verticals" like these should
improve things over a Beverage, and in many cases F/B ratio is very
important even without QRM.

73 Tom

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