Re: Antenna preamplifier

I won't use the RF preamp, it was just an idea. The reason I asked that question was because I don't quite understand how we choose gate resistor. Not because I want to build that amplifier. Even if I don't plan to build it, I want to understand it.

And no, I don't have EMRFD, I just googled it and this is the first time I came across it.

You can try it, relay isolation is important. Then some receiver components may get saturated, and it may take almost a second to recover after TX is switched OFF. This is why AGC is included in switching action.

One more thing. If I decide to build this radio in a way that mixer is one module, IF amplifier another module, IF filter another... Can I connect them with regular wire or should I use short coaxial cable? If I use coaxial cable, then I need to match all the impedances to 50 ohms instead to 200 ohms, like they are now.

Or maybe I should just build entire radio on one 16x16cm copper board. I'm using manhattan style to make RF circuits. For me it's the simplest way.



jiripolivka - are you saying that I can just put mechanical relay to switch antenna from receiver to transmitter? Is it that simple?

For switching ON/OFF the LNA you need just a simple signal relay dual-SPDT.

http://www.digikey.com/product-searc...2-amps/1049448

A simple switched ON/OFF high-IP3 HF preamplifier you can build using a BFG135 transistor:
http://www.digikey.com/product-searc...eywords=bfg135

Yes it is, only you still have to squelch the rest of receiver by AGC. Estimate with me:

- receiver noise floor is around -100 dBm (depends on bandwidth etc.)
- transmitter power, moderate 10 W is +40 dBm.
- a good coaxial switch has 60-70 dB isolation, so
- at receiver input after the coax switch you still have -20...-30 dBm when TX is ON, so the AGC must swallow it.



A good design is to build your PC boards into tin sheet boxes with tight lids, then solder them together close to holes through which live RF is going. Then maybe do not care about line impedance if lines are not longer than 2-3 cm.
Ordinary coaxial cables also leak through the external conductor. Better is to use semirigid coax, copper tube with no leaks.

I understand. But if I use the same IF amplifier (and AM, SSB filters) for receiver and transmitter, that means that the receiver is not working (except for the mixer) when the transmitter is operating. So no leaks from transmitter.

Is 15-20 meters of coaxial cable from receiver to antenna too much?