Near field to transmission antenna

Useful information regarding antennas for SDR products.
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Near field to transmission antenna

Post by ZL2BMH » Thu Aug 09, 2018 9:32 pm

Hello All.

As the manual says, don't run the receiving antenna (in my case a wire approx 10meters) near the field of your transmission antenna. So what distance would you think that would be?

I have a very small backyard that contains a 33ft mast with a 4 element Yagi, and an 80-40 meter wire (which the neighbour allows me to run half into his backyard). So I have a lot going on in the yard, therefore, I really need to know what distance I should allow.
Many thanks,

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Re: Near field to transmission antenna

Post by Roger » Fri Aug 10, 2018 4:01 pm

You can easily exceed the safe level (+10 dBm) for the front end of your RSP when your transmit and receive antenna are both located on a small residential lot. What you need is an RF limiter or SDR switch. If you search this forum you will find lots of discussions on both of these devices. There are several manufacturers that make them or you can build your own from a design I posted on this forum a few weeks ago.

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Re: Near field to transmission antenna

Post by MikeHarding » Sat Aug 11, 2018 3:38 am

Recently I camped in the (Australian) bush with half a dozen Amateur Radio friends. Peter arrived the day before me are erected his linked 40m/80m dipole about 6m above ground in a N/S direction.

When I arrived I camped about 40m east of him and erected my 80m OCF dipole 12m above ground in a N/S direction.

I had an Icom IC-7300 transceiver. We discussed the possibility of Rx front end damage from one-another's transmissions and agreed to notify the other before we Tx-ed so they could disconnect the antenna although my opinion was that the 40m distance would keep things safe.

I put a 2 way antenna switch on my radio and shorted position 2 - when not Tx-in I rotated the switch to position 2 thereby putting a short on the 7300 antenna... except... sometimes I forgot!

We operated in this way for six days and all was good and then the temptation was too much and I decided to test my theory of safe distance. I waited until Peter was having a lengthy chat with a friend on SSB 40m and running 100W then put my 7300 into circuit.

As you would expect, the 7300 bandscope indicated a massive signal but the overflow indicator (a red "OVF") only triggered on some voice peaks by no means was it regularly present. My theory was confirmed and the 7300 survived :)

Also, from home, I run the RSP1A on position 3 of an antenna switch and a 100W transceiver on position 1 of the same switch - I have been doing this for six months or more and have probably had a few cumulative hours of HF Tx in that time - the RSP1A still seems to be fine.

Having said the above: clearly it is best to subject receivers to absolute minimum Tx signals and if you have any doubts put an attenuator in circuit it *is* perfectly possible to destroy a receiver front end via overload of this ilk.
Last edited by MikeHarding on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
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Re: Near field to transmission antenna

Post by jon » Sat Aug 11, 2018 10:47 am

A big factor is the frequencies involved.

This article describes a lot of the considerations (this is a 1KW HF example) You can also see from this article that the relative polarisation of the Tx and Rx antennas is also important. You can factor you actual power level down from 1000W in the example to lower power if appropriate to get an idea of the spacings needed for safe operation (green).

There's a useful calculator here:

Always best to play it safe. You can experiment with very low levels of TX and use SDRuno's RF power measurement capability to see where the thresholds are being reached in a practical case.

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