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Re: BBQ Dish Sanity???



Hi guys

Thanks for the comments! Here are a few clarifications
and thoughts:

There were a couple of suggestions that the difference could
be dBd versus dBi. Sorry for the confusion here. I should have
stated that the gain for both antennas is spec'd in dBi.

Also, I was sent a private email from an AMSAT member who agreed
that his tests also indicated that the Model 26 was only around
20 dBi gain rather than 24 dBi as specified by the manufacturer.
This would seem to indicate a design or manufacturing problem
at Andrew rather than a specific problem with my unit.

There were also some comments about the G/T of the dish not
being as good as the horn. This might be true but in my test
I was only testing gain (not G/T) by measuring the total
received signal power (INCLUDING the noise) so if the antenna
was exceptionally noisy, it would seem to indicate that it had
MORE gain not less.

I should have noted in my original report that the noise was far
below the AO-40 beacon signal (more than 10 dB at least) for both
antennas so it would not significantly affect the gain measurement
anyway.

As Lee points out, the pigtail probably has .2 dB loss so this
would hurt the G/T by a bit. However, the difference in
gain would still be only .2 dB. What's more, the manufacturer
says the gain is 24 dB at the N-connector at the end of the
pigtail so this loss would be included already in their gain spec.


At 06:14 AM 8/7/03 +0000, Cliff Buttschardt wrote:

>Hi Tony, I ran the figures for the horn through and found the gain was
>21.5 db so your 22 db is no off!  This assumes an efficiency of 50 %,
>which is low for this large in wavelengths (4.11 by 5.286 lambda).

I assumed 55% as per Kraus so that's why I calculate 22 dB. As I
mentioned though, it hasn't been calibrated so I don't know the
exact gain. I don't think it is MORE than 22dB however, so anything
less would indicate that the Model 26 is even worse.

>  If the Andrews dipole matches the 3 foot
>dimension, then in that polarization I get 24.3 db gain again assuming
>50 percent efficiency nothing wrong here!

Yes, that seems consistent with their spec of 24 dB (+/- 1 dB) from
2300 - 2500 MHz.


>     I'd suggest measuring the relative beamwidth of both antennas.  The
>horn should be slightly wider (14 degrees) than the 3 foot dimension
>of the gridded reflector (10 degrees) for 3 db.  If it is not, then the
>feed is not positioned properly, a bad connector or something similar.
>Certainly there is little that can go wrong with the horn!! Cliff K7RR


Funny you mentioned that!

I noticed that it is very easy to find the peak in the signal with the horn.
The signal is easy to hear even off-axes, the pattern is symetric in
both Az and El and the peak is a nice simple "sweet-spot."

The dish was not like this at all. The broader Az pattern, is wide
and easy to peak but the elevation pattern, the narrow one with the dish
horizontally polarized, is not only very narrow (as expected) but the
exact peak seemed difficult to find almost like it was a flat spot
rather than an actual peak in the pattern.

I think Cliff hit it on the nose on this one - perhaps the feed is not
positioned correctly? There is no provision to adjust the feed on the
dish - it is set by the manufacturer. But, I think I could take the
screws out and move it around a little bit. Perhaps this is the problem.

I will report back if I find anything interesting.

Thank you to all and 73,

Tony AA2TX



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