Hi Scott,

I use a power-splitter (http://www.pe0sat.vgnet.nl/antenna/power-splitter/) in reverse.

So two receivers connected to the same antenna via this splitter.

This, as expected will drop down the signal with 3db but leaves the impedance at 50 ohm.

Make sure you don't, by accident, TX or enable a BIAS-T on such a setup. I have a complete separate DC supply to the pre-amp.

Antenna -> pre-amp -> coax -> power splitter -> RX1 -> RX2

73 Jan PE0SAT

--- With regards PE0SAT Internet web-page http://www.pe0sat.vgnet.nl/ DK3WN SatBlog http://www.dk3wn.info/p/ Online Telemetry Forwarder: http://tlm.pe0sat.nl/ irc://chat.freenode.net #Cubesat - Twitter @pe0sat

On 11-01-2017 02:11, Scott wrote:

Hi everyone.

I suppose that any deviation from the ideal setup of a near-perfectly matched antenna & receiver will have SOME negative effect, but for a receive-only antenna, how badly am I hurting myself to split the antenna between two receivers?

Since frequency is probably important to answer this, I’m referring to 70cm.

Also, if it matters, I have a very good preamp (SP-70 from SSB) at the antenna feeding 25 meters of LMR-400. Both receivers are SDR devices (receive-only).

When I tested against a 70cm beacon, there was some small reduction in signal strength but it wasn’t much. With voice, you might not notice but my interest is telemetry data.

Thanks for any recommendations or experiences with a similar arrangement!

-Scott, K4KDR Montpelier, VA USA twitter: @scott23192 _______________________________________________ Sent via AMSAT-BB@amsat.org. AMSAT-NA makes this open forum available to all interested persons worldwide without requiring membership. Opinions expressed are solely those of the author, and do not reflect the official views of AMSAT-NA. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://www.amsat.org/mailman/listinfo/amsat-bb

El 11/01/17 a las 02:11, Scott escribió:

Hi everyone.

I suppose that any deviation from the ideal setup of a near-perfectly matched antenna & receiver will have SOME negative effect, but for a receive-only antenna, how badly am I hurting myself to split the antenna between two receivers?

Since frequency is probably important to answer this, I’m referring to 70cm.

Also, if it matters, I have a very good preamp (SP-70 from SSB) at the antenna feeding 25 meters of LMR-400. Both receivers are SDR devices (receive-only).

Hi Scott,

A perfect splitter introduces a loss of 3dB, since half of the signal goes to the other receiver. A real world splitter will have a slightly greater loss due to imperfections, but something like 3.1dB or 3.2dB is usual, so I'll call that 3dB for the sake of the argument.

If you have no preamp, this raises your noise figure by 3dB. However, since you have a preamp before the splitter, you have to divide by the gain of the preamp. Say your preamp has a gain of 20dB (which is typical). Then your noise figure is only raised by 0.03dB, which is tiny.

The important question is how much signal-to-noise you lose because of this increase of 0.03dB in noise figure. This depends on your antenna temperature (which is the amount of noise that your antenna grabs from the environment). Noise figure is normalized at 290K, so if your antenna temperature is 290K you lose precisely 0.03dB of SNR. Your antenna temperature is almost never 290K. At 2m it is likely that the antenna temperature will be much greater than 290K, on the order of thousands of K. Therefore, your losses in SNR are much smaller than 0.03dB. On 70cm and higher, the antenna temperature can be lower than 290K (much lower on the high microwave bands), especially in quiet rural zones. Therefore, your SNR losses will be much greater than 0.03dB but still a fraction of a dB, so you won't even notice the losses.

These are the exact calculations (by the way, the same calculations can be used for coax losses and any other sort of losses after the preamp). In layman's term it's much simpler: you lose (almost) nothing, because your preamp provides enough signal gain to feed both of your receivers with adequate signal level, despite the fact that the signal power is split in half.

There might be a problem if the gain of your preamp is especially low (say 10dB) and you're in a very quiet area.

73,

Dani.