TX near SDRPlay RSP1A with HTs

[Just_Listening]

Hi everyone.

I have just purchased the RSP1A and it's working very nicely. For now, I am using a short piece of wire as an antenna that I have connected to it and was surprised to hear VK4 and VK1 from my QTH in Melbourne (VK3) during the "Trans-Tasman" contest last night. Very sensitive.

I have read the specs and know that the RSP1A can only tolerate 0 dBm continuously (+10 dBm briefly) so I know not to have an antenna connected while I TX with my IC-7300. I have been reading through posts on this forum about protecting the RSP1A in those situations.

But like most Hams, I have a couple of 2m/70cm HTs that put out 5w which I use inside, usually in the same room as where I will be using the RSP1A. I also run an IRLP node (simplex mode) that puts out 2w when it's radio goes into TX.

My question: Just to be safe, do I need to take the antenna off of the RSP1A when I want to use the HTs and IRLP node?

The reason that I ask is that I have my Uniden TrunkTracker hand scanner scanning the 2m/70cm repeaters most of the time and it doesn't seem to mind me using the HTs.

Thanks -- Pete

[Roger]

Hi everyone.

I have just purchased the RSP1A and it's working very nicely. For now, I am using a short piece of wire as an antenna that I have connected to it and was surprised to hear VK4 and VK1 from my QTH in Melbourne (VK3) during the "Trans-Tasman" contest last night. Very sensitive.

I have read the specs and know that the RSP1A can only tolerate 0 dBm continuously (+10 dBm briefly) so I know not to have an antenna connected while I TX with my IC-7300. I have been reading through posts on this forum about protecting the RSP1A in those situations.

But like most Hams, I have a couple of 2m/70cm HTs that put out 5w which I use inside, usually in the same room as where I will be using the RSP1A. I also run an IRLP node (simplex mode) that puts out 2w when it's radio goes into TX.

My question: Just to be safe, do I need to take the antenna off of the RSP1A when I want to use the HTs and IRLP node?

The reason that I ask is that I have my Uniden TrunkTracker hand scanner scanning the 2m/70cm repeaters most of the time and it doesn't seem to mind me using the HTs.

Thanks -- Pete

If you want to be safe you will take the antenna off when you transmit in the same room as the RSP1A with a short antenna attached to your HT. If the antenna is outside and brought into the radio room with coax you will not have as much signal at the antenna input because the Rx antenna is further away from your HT and you might get away with it. Another option is to build or buy an RF protection box for your RSP1A but make sure it has low insertion loss at the frequencies you intend to receive on. Your Uniden probably has more front-end protection than the RSP1A. The reason is that the RSP1A covers a wide frequency range (100 kHz. to 2 GHz.) and a trade-off had to be made for protection vs. sensitivity at the higher frequencies.

The gain of the Rx and Tx antennas, the transmit power and the distance will determine how much power comes out of the Rx antenna. Here is an example. Lets say both antennas are 2 M "rubber ducks" and they both have a gain of -6 dBi . If the HT power is 5 W on 144 MHz. and the antennas are 3 feet (1M) apart the received signal strength is 10 dBm. At 10 feet (3M) the Rx signal will be about 0 dBm. If your receive antenna is a ground plane with 0 dBi gain the received signal levels will be 6 dB higher

[Just_Listening]

Thanks for that advice Roger. Much appreciated.

I thought that would be the case. I found an online "watt to dBm calculator" which gave 1w as 30dBm and 5w as 36.9897dBm, which is way over the safe limit of RX input for the RSP1A.

Looks like I will be needing to build one of those RF protection boxes like the MFJ-1708B-SDR.

Cheers -- Pete

[Roger]

Thanks for that advice Roger. Much appreciated.

Looks like I will be needing to build one of those RF protection boxes like the MFJ-1708B-SDR.

Cheers -- Pete

The MFJ box is not meant for protection from your HT. It is when you wish to share an antenna with a transceiver. What you need is much simpler and is an RF limiter. You can build your own (I gave a circuit in another post) or buy one on ebay for $35 USD. DX Engineering and Array Solutions both sell high quality units for around $70.

[Just_Listening]

G'day Roger.

I take your point about the HTs and any other TXs near the RSP. Because I want to use the RSP as a stand-alone RX, I will give your RF Protection Circuit a go. It looks easy enough to build.

Thanks again.

Cheers -- Pete.