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5/8 wave GP antenna for sats (was Re: ...)



Hi Tim,

I hope my comments are constructive and help explain what is happening. If 
you need more information I'll be happy to help out with any additional 
information you might need.

First off, several things pop out at me to suggest where the problems are:

  - A 5/8 wave ground plane antenna has very good gain, but unfortunately 
has a radiation pattern that is designed to travel along the ground.  So 
when you have a high pass you are actually losing gain, not improving it.

- A ground plane antenna is omnidirectional, so although the signal of the 
satellite is coming from one direction, the noise is coming from all 
directions.  So the signal-to-noise ratio is much higher.

  - AO-51 transmits with 500 mw.  UO-14 was nominally 4 watts or 9db 
greater.  So something that "heard" UO-14 at S2 or S3 will not necessarily 
be able to hear AO-51.  Good rule of thumb - if you can hear SO-50 (250mw) 
at S1, you will still only have S2 for AO-51.  If you can't hear SO-50, you 
might not hear AO-51. I always recommend to people to start off listening 
to SO-50.  If you can hear it well, you are in business.  If you can't, you 
have your work cut out for you.

  - RG-8 is lossy at 436MHz - I don't have the exact number handy but I 
think it's about 4.9db per 100 ft.  If you are using RG-8X (popular on HF 
and 2M ) at 436 the attenuation is 8.1db per 100/ft.  9913 on the other 
hand is 2.6db per 100 feet.  LMR-400 is 1.9db per 100 feet.  I use LMR-400 
Ultra Flex for receive (436MHz) and 9913 for transmit (145MHz).

- The Preamp in the 706 won't do much good because it is located in the 
radio, not at the antenna.  So it is amplifying signals after the cable 
losses have been factored in and won't overcome the losses in the cable.  A 
mast mounted pre-amp will help, but I don't know how much extra signal gain 
you will get out of an omni-directional ground plane versus just amplifying 
ground noise.

Your comment about the net is familar because I hear from people all the 
time that "the setup must be good because I worked a repeater xx miles 
away".  This is very misleading for several reasons.

  - The repeater is probably located no more than 15 degrees higher than 
you are.  Your ground plane (ie, radiation pattern along the ground) is 
optimized for gain in that narrow elevation pattern.

- The repeater antenna is likely to be optimized transmit along the ground 
as well.  You are located in a very optimized point in it's radiation 
pattern so the gain is very high.  AO-51 is not optimized to transmit along 
the ground (obviously - it couldn't be.)

- The repeater is probably a 2m repeater - fewer losses.  If it was a 70cm 
or cross band repeater that would be a better scenario, but still not the 
same as a satellite.

- The repeater is likely to be transmitting anywhere from 50 to 150 
watts.  (If it's a simplex net it's probably safe to say that the stations 
were running 50 watts high power and also using high gain ground planes or 
collinears.)

- AO-51 is 100+ times weaker and probably 100+ times farther away.

My recommendation would be to improve your antenna system by using either 
eggbeater antennas (which will absolutely require preamps and won't help 
the signal-to-noise ratio) or directional yagis (which would still benefit 
from mast-mounted preamps but will improve the signal-to-noise ratio.)

I use two small Diamond yagis and they work fine 96+% of the time.  They 
are by no means ideal compared to the circularly polarized yagis but they 
are cheaper by quite a bit. If you use yagis you will probably need 
rotators for elevation and azimuth, but there are ways to work around that 
if you are willing to do some legwork and/or make some compromises.

73,

Emily


At 11:33 AM 1/25/2005 -0500, you wrote:
>Hi Folks:
>
>I am experiencing extreme frustration trying to hear AO-51
>
>Let me describe my setup which, I realize is not great for satellite work 
>(which is why I'm only trying to hear the blasted thing)
>
>Radio - IC706MKII
>Antenna 5/8 wave Bottom loaded groundplane at about 15 feet
>the only pre-amp I have is the one in the radio, and the COAX is basic RG 
>-8 i think
>
>
>I have been trying off and on to hear AO51 -- even just a gradual slight 
>silencing of the static would make me happy just to know I can hear it.
>I mean guys are working the thing with HT's and Rubber duckies without any 
>trouble, and I used to use the same antenna to listen to UO 14 on my vx5R.
>
>Last Night about 9:30 or quarter to 10 -- I dont have my numbers in front 
>of me, I was listening from 435.290 to 435.310 during a 69 degree pass 
>here in the cleveland area and I heard nothing. My keps are only about 2 
>days old, I use nova and just had it do an automatic download from 
>whereever it defaults to to get them (celestrack I think). I didnt see 
>anything on the schedule page about the spacecraft being off.
>
>I really dont know where the problme is.  I had just used the radio and 
>antenna system earlier in the night for a net, so I know that its all good.
>
>Please throw some suggestions out here, I know I really dont have a prayer 
>of making it into the spacecraft without el-az antennas and true full 
>duplex, but I would think that I should be able to at least here some of 
>the downlink from it.
>
>Thanks
>TIM
>W8TAH
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---------------------------------
W0EEC - CM87tm
AMSAT Area Coordinator - San Francisco Bay Area
http://www.projectoscar.net    http://www.PlanetEmily.com

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