Antenna preamplifier

Dan Mills said:

Seriously, on the HF bands you do not need a preamp, and adding one will make things worse....

Start with a 14MHz BPF to remove as much of the out of band energy as you can, then a mixer (with diplexers on all ports to provide correct wideband termination is a nice touch, but you can add those once it works), then place your IF filter, then the IF amp...

Doing it that way you expose each stage to as little energy as possible, minimizing intermod products, you dont want the IF amp exposed to the whole 14MHz band if you can avoid it, which is why it goes after the main selectivity.

For TR switching, some combination of relays and diode switching is normal, have a look at the Picastar for an interesting bidirectional amplifier.

Do you have a copy of EMRFD?

Regards, Dan.

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I14R10 said:

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.

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

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jiripolivka said:

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.

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