Hi is there a tx/rx switch for the?lz1aq amp so I can use my EFHW for tx and the?lz1aq loop antenna for rx on a HF radio that doesn't support a separate rx input.

Kind regards?
Steve
VK4SJB?

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? 73
??? Bob? KD8CGH

Hi Bob, the diagram on that page only shows a SDR and Transceiver sharing a single antenna. It doesn't seem to separate 2 antennas one for tx and one for rx.

Or you could easy make one..relay..transistor, cap or 2, resistor or 2.. little metal box, 3 so239’s and 12v in.oh and a phono socket.

Would need to go in non 12v powered rx line…

Simon

开云体育

I have the DX Engineering switch that adds a antenna port for receiving. It works super but is a pricey bugger. ?I am using a simple 2 position coax switch to use 2 different radios.?

On Tue, Jan 11, 2022 at 04:40 PM, Bob Benedict, KD8CGH wrote:

The isolation between transmit and SDR receive is not good enough on this product.? It will work OK for QRP but not with a transmitter using more than 10 watts.?

Just some answers:
1.? >>The isolation between transmit and SDR receive is not good enough on this product.? It will work OK for QRP but not with a transmitter using more than 10 watts.?

1.?????
? I think that there is misunderstanding :???? -30 dBm is the maximal signal level at the SDR input when 100 w TRX at full power is used.??? 100 W = +50 dBm so the insulation is? 80 dB.? This is at the highest frequency. For example at 1.8 MHz the insulation is above 90 dB. ?I will change the documentation to make that clear.

2. ? A separate RX antenna can be used just do not insert the TX antenna cable into Antenna switch module. In this case the Antenna switch will act as an extremely reliable RX protector with double protection.? The relay is grounding the input + ?heavy duty limiter which can withstand even 100 W for hundreds of milliseconds. But normally ?I have additional small relay which can switch the input of the SDR either to Antenna switch (to use TX antenna for receiving) or to RX antennas. Our active antennas have ?protection limiters (2 levels) ?. They limit the level at the SDR input to 6 V pp – no limiter, 4.2 V pp (limter 1)? 2 V pp (=10dBm limiter2). So in this case the SDR is protected both from TRX and RX antennas.

73 lz1aq

On Wed, Jan 12, 2022 at 01:40 PM, чавдар левков wrote:

?
? I think that there is misunderstanding :???? -30 dBm is the maximal signal level at the SDR input when 100 w TRX at full power is used.??? 100 W = +50 dBm so the insulation is? 80 dB.? This is at the highest frequency. For example at 1.8 MHz the insulation is above 90 dB. ?I will change the documentation to make that clear.

Thank you for the clarification on TX-SDR isolation.

It would have been nice if you has a hybrid splitter (for receive) in the design instead of using a T connector and a 3 .7 db pad on the receive (for impedance isolation). MFJ uses one in their MFJ-1708BSDR and it provides isolation between the SDR and the transceiver during receive operation.?

Some comments on the use of a T-connector during receive. A T-connector is a poor way to parallel two receiver inputs together. Doing it this way can lead to a number of problems including:

• The input impedances of the two receivers are rarely 50 ohms and the signal will not be split equally between the receivers. If they are both 50 ohms there will be a 3 dB drop in signal power into each. If one is 50 ohms and the other is less than 50 ohms the lower one will get more power and the 50 ohm input receiver will have much more than 3 dB loss.? This is? particularly evident when the two radios are on different bands and the front-end filters are not operating in their passband.

• Even when both receivers are 50 ohm input impedance they are 25 ohms in parallel and this will result in a mismatch with the circuit or antenna providing the receive input signal and result in less than maximum signal power transfer.

• Spurious signals present on the antenna input port of one receiver (like local oscillator feedthrough) will directly couple into the other receiver and degrade it's performance.

• If a bias-t port is used on the SDR and the bias T is accidentally enabled approximately 5 volts will be injected into the front end of an expensive ham transceiver. This can cause considerable damage if it does not have DC blocking at the antenna input.?

The best approach is to use an RF splitter to split the signal between the receivers. This is a passive device which has about 3.8 dB of power loss and at least 20 dB of isolation between ports. This loss is not significant on HF (below 15 MHz.) because the signal and noise are both lowered by the same amount so the SNR stays the same.

Roger