Thursday, February 17, 2011

Just an Idea

From VHF and UHF operations we know to use preamps at the antenna feedpoint as to improve SNR. SHF and EHF amateur radio operation is usually done by having the transverter at the antenna's feedpoint, since losses in the transmission line would eat up the signals completely...

So, what's the idea? Well, in QRSS, we also hunt for the best possible SNR. So, why not moving half or more of a QRSS station's circuitry to the antenna?
Half? Yes, I would prefer/advice to have frequency determining blocks, i.e. oscillators, in a controlled environment, e.g. the shack.

The basic trick on all of the following would be feeding DC from the shack to the device at the antenna, whatever the device will be. Feeding is done in the well known way for active aerial or mast-head preamps.

Easy things first, lets have a look at transmitters. The only bit that needs to be at the antenna feedpoint would be the final amplifier. The only thing to watch out for would be the Ohmic loss of the transmission line, since the final stage will require some current. If one cares about the voltage which the final is driven from, one should take this transmission line DC resistance into consideration.

Reception is somewhat more challenging. Why do it in the first place? A preamp could do that. Yes, it could. However, some designs (e.g. w/ the NE612) do not really need pre-amplification...
Here's a look at the different cases:

Direct Conversion
In this case, the LO would be in the shack, and the whole rest of the receiver remote at the antenna location. It should not pose any problem to combine/split LO-RF, RX-AF and DC. However, the supply DC can carry mains hum residuals. A decent high-pass will be necessary before the AF can be fed into any sound-card.

Supersonic-Heterodyne (Superhet)
In such a scenario, front-end, mixer and IF filter (xtal-ladder) would be distal. LO, BFO, product detector and AF-amp would proximal. It is conceivable/advisable to add a second xtal-ladder to split IF and BFO frequencies. The downside would be the more complex frequency splitting at the distal part. A crystal filter would come handy here, and therefore, a "cheap crystal combination" would be recommendable for such a design, however tempting a DDS design would be.

Now to the tricky stuff... RX/TX-toggle could possibly done by polarity reversal, e.g. by diodes and relays.
I would like to leave it to your imagination what would be possible...

This may not be necessary to think about at all...
However, in case of reception, having all the low-noise stuff at the distal location, one may consider using cheap twin lead cable to connect the remote head with the shack.
For transmission, the obvious advantage would be that the final could be matched to TX-aerial, whatever it may be in balance and/or impedance.

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