Log Periodics do work well as a dish feed.
I agree, not quite as well as an optimized scalar feed, but the difference
is about 1 dB on a small dish. And you get 3 or 4 ham bands.
The feedpoint of a dish is a theoretical point and usually calculated as though the
dish is used at optical frequencies. In testing well over 100 dishes, the optimal
position for the feed is almost always closer to the dish than the calculated feed point.
In part this is because the feed is not an infinitesimal point.
Yes, the phase center of a log periodic does move, but it doesn't move very much.
When optimized at the highest frequency of use, the phase error at the lowest
frequency of use is quite minor.
Another option is using a Vivaldi antenna for the feed. I like to think of a Vivaldi
and a Ridged Horn without the horn. Phase center is pretty constant with
frequency. The Vivaldi has about 2 dB more gain than a Log Periodic and would
be more suitable for most offset fed dishes.
From: "g7jtt@... [ukmicrowaves]"
Sent: Sunday, May 13, 2018 7:44 AM
Subject: [ukmicrowaves] Re: Advice
just came across this site which has a multi band ring feed http://gvnweb.ddns.net:8081/dish1.html
and links to some design software as well. the only thing is this is used on a prime focus dish are these feeds ok to use with an off set dish ?