Can a homebrew QRP TX share an antenna with a cheap general coverage receiver?

Hi all, I hope this thread is not too off topic.

I wonder if this setup is possible and if one has actually done it.

So imagine you have a relatively cheap portable all mode all bands receiver, like the Tecsun PL 880 for example; an HF antenna and a homebrew QRP trasmitter (or transceiver, for example the Pixie or similar).

Would it be possible to use the bigger antenna on the receiver when not transmitting and then have the transmitter to use the antenna, while the receiver uses its own whip antenna at the same time?

The reason I want to do this is so that I can hear myself keying on the receiver.

An alternative to this setup is to use a regenerative receiver instead of the Tecsun.

I know nothing of antenna switching between separate RX and TX so forgive the silly question.

Thank you.

[kellys_eye]

Normally you would disconnect and ground the antenna input of the receiver using a keying relay. At the same time the antenna is changed over to the transmitting set.

'Break-through' will give an audio signal in the receiver.

Having some form of receiver input protection is preferred - back-to-back diodes across the receiver input for example - depends on the power levels in use.

[MrBungle]

That is roughly how W7EL optimised QRP transceiver works. See output stages here:

http://www.arrl.org/files/file/Techn...f/93hb3037.pdf

Although this sneakily shares the output filter network between TX and RX.

I plan to do exactly this when I get my intermediate.

[Andrew2]

Ottavio, I'm a little confused. You say ' I want to do this is so that I can hear myself keying on the receiver.' But you'll be able to hear your own signal whether or not you have your big antenna connected.
In fact, your own signal will be so strong your radio will probably jump off the desk and hide!

Originally Posted by kellys_eye

Normally you would disconnect and ground the antenna input of the receiver using a keying relay. At the same time the antenna is changed over to the transmitting set.

'Break-through' will give an audio signal in the receiver.

Having some form of receiver input protection is preferred - back-to-back diodes across the receiver input for example - depends on the power levels in use.

Thanks. Can you clarify what 'Break-through' means? Is this standard behaviour for a TR switch?

Originally Posted by Andrew2

Ottavio, I'm a little confused. You say ' I want to do this is so that I can hear myself keying on the receiver.' But you'll be able to hear your own signal whether or not you have your big antenna connected.
In fact, your own signal will be so strong your radio will probably jump off the desk and hide!

Correct me if I'm wrong. I won't be hearing me if the RX is muted. Or will I?

The standard TR switched mute the RX. I just want to switch the external antenna between RX and TX but not mute the RX.

Sorry if I'm saying something silly here. This is ground zero for me.

Originally Posted by MrBungle

That is roughly how W7EL optimised QRP transceiver works. See output stages here:

http://www.arrl.org/files/file/Techn...f/93hb3037.pdf

Although this sneakily shares the output filter network between TX and RX.

I plan to do exactly this when I get my intermediate.

Thanks for the link. Please give me some time to digest it.

[Andrew2]

Ottavio, if your receiver is a 'world band' portable it will not have any way of muting the receiver. The best you can do is to use a relay to disconnect the antenna and short the input of the RX to earth. This will do enough to avoid damage, but it will not 'mute' the receiver. Your transmit signal will still be *very* audible. Hope this helps.

[Station X]

Seems to me that all you need is a changeover relay in the antenna feeder. The common contact goes to the antenna. The normally closed contact goes to the receiver. The normally open contact goes to the transmitter.

This means that there is no possibility of connecting the transmitter output directly to the receiver. However you will need to make sure that any sequencing arrangements prevent the transmitter operating into an open circuit. ie until the changeover relay has operated the transmitter cannot emit RF.

However I suspect the above arrangement will result in receiver overload during transmit.

Thanks @Andrew2 and @Station X.

So, in other words, is this kind of setup acceptable? Maybe using a portable receiver as a part of a QRP rig is not a good idea.

[G6Tanuki]

The "right" way to do this is to use a double-pole changeover relay. One pole switches the antenna between the receiver and the transmitter; the other pole puts a short across the receiver's input when on 'transmit'.

Despite this, I suspect you will have problems: even with the receiver's antenna-terminal shorted to earth it will 'see' much more RF signal than it was ever designed to handle - so its AGC circuit will be overloaded and the receiver's gain will be turned down in a spectacular way. When you switch back to receive it could take a few seconds for the accumulated charge on the AGC circuit to dissipate before your receiver will become sensitive-enough to hear anything.

Uisng a cheap broadcast-receiver is really not the smart way to be getting on the HF bands.