[amsat-bb] Re: Polarity Switcher Relay - repost
bob at evoria.net
Wed Sep 5 12:48:28 PDT 2007
--- Bruce Robertson <broberts at mta.ca> wrote:
> In your previous letter, you describe the antennas as "turnstile on 2m and
> a turnstiled moxon on 70cm"; are these fixed, like the moxon Cebik
> recommends, or are they rotating beams?
Actually, the 70cm is just like Cebik published in QST. But, for some
reason, I don't get as good performance on the 2m version, so I switched
that to a regular turnstile 3/8 wavelength above a layer of Reynold's
Wrap. I've also managed to work a number of 2M SSB stations on
that antenna. I don't have any rotators; they're just pointed straight up.
> As you probably know, depending on the roofing material, you are suffering
> a not inconsiderable loss on 70cm from the signal passing through the roof.
> Last time I looked into it, I figured on about 2dB for my shingled wood roof.
I didn't know how to figure this, but I figured it would be 1 or 2 db. Winter
snow is going to be unworkable, of course, so the plan is to move them
One of the biggest negative impacts I've had is when I added the 70cM brick.
It has about a 6db loss on receive and it took me awhile to figure that out.
> If you are certain you don't want to try mode B, you could even get a
> receive-only 70cm preamp,
Actually, I have made a handful of mode B contacts on both AO-7 and VO-52.
I've just gotten kinda sidetracked with this 70cm polarization thing and I
figured anything I did that improved reception (other than using a preamp
of course) would improve my mode B uplink. I've listened to a few contacts
on AO-7 mode A recently, but for some reason, neither of my 2M antennas
has enough oomph to get me an SSB signal that I can hear - even with the
80 watt brick turned on. I'll get that worked out, eventually. I really want
to have some mode A QSOs using my KWM-2A on downlink.
>On 70cm CW you can also try
> LO-19, which has a booming signal. I find it easier to use these to assess
> the effectiveness of an antenna than to use the FM birds.)
I'll give that a try. One of the problems I have is that I don't have anything
to use as a baseline. I recently found a very strong digital repeater on
433.7xx and I used the signal from that every 20 seconds to discover the
amount of loss in my 70cm brick on receive. Normally, for alignments, I use
the IF bleedthrough from my VX-5R in WFM mode.
> Yep. I just worry that you're not getting enough out of the antennas you've
> already built. Quite honestly, I'd rather spend an evening doodling on the
> birds with my indoor 1/4w whip and a preamp on 70cm than with my outdoor
> beam and no preamp!
I'll squeeze everything out of the antenna I can and then I'll add preamps.
But, I really want to build them myself if I can get over the engineering/
parts list issues.
> However you proceed, *please* keep reporting your results to the list. I
> think the best thing we can do as a group is support each other in
> antenna-building and -experimentation. If you can demonstrate a reliable
> station with attic antennas, I think you'll be breaking some ground for all
> of us.
Will do. I've made a lot of contacts on the FM birds with my setup. Of
course that's just a callsign and a grid-square. But, the thing about doing
this is that I've had to optimize everything, including squeezing the last bit
of S/N out of this old FT-726R that I could tune for. For that, I resorted to
the IF bleedthrough mentioned above and tuned for lowest noise. But, I have
to admit that the difference from that to max signal might have been only
wishful thinking. ;)
Bob - AE6RV
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