[amsat-bb] Re: Station not coming together - the full post (sorry for repost, reply to this)

Bruce Robertson broberts at mta.ca
Tue Mar 6 20:46:48 PST 2007

Quoting Jonny 290 <jonny290 at gmail.com>:

> LONG LONG LONG post ahead.
> OK, the situation is: I'm building up a satellite station to get
> familiar
> with LEO sats and to get ready for the phase 3E fun to start soon.
> Here's my
> station, and I'll post my thoughts after that.
> The "too long, didn't read" version: I've built good antennas, fed them
> with
> good feedlines and into a supposedly good receiver, but I just can't
> keep
> solid copy on LEO birds, and cannot get a QSO. I am jealous of the
> "Satellites worked with HT and 5/8 wave" articles, and want to figure
> out
> what the weak link in my station is.
> RX path:
> Eggbeater II antenna mounted at 15 feet. This is built to the K5OE
> Eggbeater
> II spec (http://members.aol.com/k5oejerry/eggbeater2.htm) using RHCP
> but
> uses a 100 ohm phasing line using two 50 ohm coax lines done in
> parallel,
> used in a balanced configuration, instead of 93 ohm RG-62. Antenna was
> 'redesigned' in MMANA-GAL to match exactly 100 ohms and to use 6mm
> diameter
> copper tubing instead of AWG 12 house wire. It is peaked for SWR < 1.1
> at
> 436 MHz, and is less than 1.5 at 435.0 and 437.0.
> Our lot is lightly treed, but there is no wood (natural or otherwise)
> within
> six feet of this antenna. A good chunk of the sky, mainly northwesterly
> looking, is partially obscured by a large tree, but right now the leaves
> are
> off and it shouldn't kill the signal that much...should it?
> This antenna is the secondary antenna on this mast and is mounted on a
> 3
> foot PVC stand-off pipe. The primary antenna is a Diamond F22A mounted
> at
> approximately 18 feet. It has a 10 foot vertical element and three 1/4
> wave
> horizontal radials. It is not used in the satellite station but I wanted
> to
> describe it, in case it is interfering (I suspect it is.)
> Eggbeater is fed through29 feet of 213 spec coax (5.1 dB loss/500 MHZ)
> terminated in well done PL-259 connectors. I have verified this
> particular
> cable's loss at less than 2 dB at 432 MHz, using my Icom PCR-100's
> accurate
> S-meter and a known signal source.
> My latest upgrade: RX chain goes into a homebrew bypass relay / Ramsey
> preamp box (the relay is DPDT and switches the preamp in or out of the
> RX
> signal path). The preamp provides approximately 18 dB of gain when
> inserted
> and using a test signal around 432 MHz. It has proven somewhat
> beneficial,
> giving me 2-3 S-units advantage at certain points from the LO-19 CW
> beacon
> (Which I recently received at up to S9 with the preamp on a ~60 degree
> pass,
> a new record so far).


Thanks for submitting your antenna system to the scrutiny of this list. For
many of us, planning and organizing these systems are good fun. I have some
direct experience that pertains to some part of your system; I hope those
with other experiences will concur or debate.

As you probably already know, it is considered a requirement of satellite
work that a low noise preamp. be placed before your receiver, especially on
70cm. One measurement that indicates the quality of this is its gain, and
you have indicated the Ramsey unit's declared value in this dimension.
However, an equally important measurement of quality is the noise figure. I
believe most advise that this be below 1 dB, and the lower the better. The
math on this issue is really very revealing, and I'll leave it to you, if
you're interested, to pursue it with Davidoff, _The Radio Amateur's
Satellite Handbook_. However, it's important to note that a high-gain
preamp with an inappropriate noise figure likely will do nothing to improve
your reception of satellite signals.

In hopes of getting an inexpensive preamp for my satellite work, I built
the 2m version of the Ramsey preamp you described. I quickly concluded that
it didn't do what I wanted. Now that I own a mast-mounted ARR preamp., I am
certain that the unit I built doesn't have the appropriate noise figure.
Now 2m is way less touchy in all this than 70cm, so I have to say that I
suspect your preamp. is simply not up to the task. I highly recommend you
buy a known quantity for this part of your system: Advanced Receiver
Research and SSB USA are both well-liked. Once you know what one of these
can do, homebrew to your heart's content until you get something that
performs as well or better. Then sell your commercial preamp on this list!

Secondarily, it is very important that the preamp be as close to the
antenna as possible. That 2 dB of loss on the cable is killing you. You can
effectively negate it by putting the preamp before it; but you never get it
back if you put the preamp after it. Again, I recommend playing with the
simple formulae and seeing for yourself how the configurations of equipment
are very important.

I know mast-mounted preamps are a big pain; and I know bias tees are not
fun. But given your cable and your antenna, you don't have a choice, you
really don't.

Finally, your antenna. This is one I would very much like to build myself;
it seems very carefully designed. However, due to its slightly complex
construction I have my doubts about using it as a first satellite antenna.
I have advocated elsewhere that everyone have on hand a simple 1/4 wave
vertical to use as a known-good antenna. Outside, tilted and in the clear,
it may well only pick up very little intelligible audio of an FM bird's
pass with an FM-only radio (a CW one will get the warbling tone quite
nicely), but it should quiet the radio a bit and give some general
indications of life. You should then expect your egg-beaters to be better.

By using fixed antennas, you are, of course, limiting your gain. That means
everything after your antenna has to be spotless: no 2 dB here, 2 dB there.
Unlike in HF, where the RF sometimes just gets through, here you're dealing
with the Hard Numbers, and they will crush you into silence. 

> Receivers:
> IC-208H dual band radio, rated at <0.18 uV for 12dB SINAD
> OR
> PCR-100 computer controlled receiver, rated at <0.34 uV for 12dB SINAD
> The TX setup is currently an IC-271 pushing from about 4 to 38 verified
> FM
> watts into a simple RHCP turnstile, recently built. It is mounted on a
> slightly shorter PVC mast about 10 feet away from the "big mast". It is
> the
> only antenna on this pole, and is fed through 27 feet of 213 spec coax.
> Peaked at 145.8 MHz with SWR less than 1.1 . I know the turnstile
> pattern
> has most gain directly overhead and has some shortages towards the
> horizon,
> but I have plenty of theoretical EIRP to get on the LEO sats, I believe,
> and
> I'm not worried about the TX path right now.

Don't worry about the uplink. You're fine there.

> I'm just not getting the signal strength and intelligibility that I
> expect
> from this setup. For example, there was a theoretically perfect SO-50
> ascending pass today at about 5:15 PM Central, 15 minutes plus duration
> and
> it peaked at almost 80 degrees elevation.  I received the signal
> faintly
> starting at 2 degrees, and it steadily improved to a 'fluttering S3" at
> about 8-10 degrees. I attempted to come back to a station that finished
> a
> QSO, because I thought it cleared - turns out,the signal DIED. I could
> just
> barely track it for the rest of the pass, even when it was almost
> directly
> overhead at less than 900 miles altitude.  Gave up when it was five
> minutes
> before LOS, with no contact made.
> So, given my station description, what am I facing? Is my Eggbeater's
> 'overhead 10dB null' design killing it? I would think that, as designed,
> the
> greatly decreased path loss as the bird comes overhead should compensate
> for
> this shortage, and then some.
> As mentioned earlier, I'm also concerned that the Diamond F22's radials
> are
> destroying the radiation pattern of this antenna. I'm in a rent house
> and
> have to conserve masts as much as possible, but if it's killing me, I
> can
> move the satellite antennas to a ~12 foot PVC mast instead of trying to
> piggyback them on my metal vertical mast.I've copied beacons and
> various
> carriers from birds as low as 0.5 degrees, so I know the antenna 'gets
> out'
> okay at lower elevations. It's the high elevation periods that are
> disappointing me.

I honestly don't think this is your first worry. It's all about the preamp,
mate :-)

> How does my IC-208H compare to more refined receivers? Is it too deaf?
> I've
> considered feeding my IC-735 HF radio with a 435 MHz receive converter.
> This
> would allow all mode 70cm reception with a very good radio (comments on
> the
> Hamtronics converter boxes would be great). I'd love any comments on
> the
> Hamtronics receive converters from those who have tried them out.

This is super-great fun. I bought one of these when I started out and used
it with my IC-728. FO-29 is an amazing bird. Worth the price of admission.
One warning, though. Be sure the converter can do 435 MHz. Mine was 432
only, and I had to buy a new crystal for it. Alas, I sold it this summer in
the Great Kenwood Purchase of 2006.
> I'd appreciate any comments and advice from the great minds on this
> list.
> Thanks for taking the time to read up and give any advice or comments.

No great mind; just a fellow-traveler.

73, Bruce 

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