Wellbrook Communications RX-only loop Excess Power Detected error

Eric Macauley

I am running a Flex 6400 with a 20m inverted-V dipole apex at ~20ft connected to ANT-1, and just recently have added a Wellbrook Communications ALA1530LNPro receive only loop to the RX-A input mounted vertically at about 10ft. Due to property limitations, I'm not able to get a large amount of physical separation between the two antennas (~20ft of horizontal distance from the bottom of one leg of the dipole to the loop). Depending on where I have the nulls of the loop pointed, when transmitting on the dipole at 100W I'll often get the "Excees Power Detected on Reciver" error message from SmartSDR when receive antenna is set to the loop on RX-A.

Short of getting more antenna separation, what other things can I do to alleviate this problem? The loop antenna has an antenna interface in the shack that supplies 12V on the feed line to power a preamp in the antenna, is there a way I could switch the 12V off/on quickly enough during T/R transitions?

Lawrence Kellar KB5ZZB

Does the well brook have a receive isolation switch like the Pixel and other Magnetic Loops? I have my pixel hooked up to turn off during transmit

Eric Macauley

Unfortunately it does not. The "Antenna Interface" box as Wellbrook calls it just has coax in/out and DC power connection, no key input to tell it when radio is keyed down to switch off the power to the antenna's preamp.

Tom

Possible solution:    http://ok1rp.blogspot.com/2015/03/front-end-saver-for-lb-antennas.html

Lawrence Kellar KB5ZZB

http://www.mfjenterprises.com/Product.php?productid=MFJ-1708SDR

Lawrence Kellar KB5ZZB

Would the above work?

Eric Macauley

I think that would definitely work for protecting the radio receiver and prevent the excessive power error message failsafe. But I am wondering if leaving the antenna's active elements powered up during transmit could do damage to the antenna components, that's why I was originally thinking about how I could power-down the antenna preamp during transmit instead and **** two birds with one stone.

David Salomon

Eric -

You just described my situation to a tee.  I have a LZ1AQ receive loop location about 20 feet from an OCF dipole.  I use this in front of the receive loop: https://www.dxengineering.com/parts/dxe-rtr-1a.  That particular unit is no longer available, but DX Enginnering has this: https://www.dxengineering.com/parts/dxe-rtr-2 and this: https://www.dxengineering.com/parts/svp-sv-fesss.  The front end saver is a disconnect only device, whereas the RTR units do a disconnect on keyup and have a full time limiter to ensure the front end does not get overloaded with strong signals not from your own transmitter.

I've used the RTR-1A for many years and never had an RF related issue.  I use it to feed my loop signal to an amplified splitter, then to tmy 6600M RX-A and several SDRs.

73 - David, AG4F

Wayne

I thought as long as full duplex was not selected it was protected automatically on single scu rigs. That is what I thought I seen in the manual.

Eric Macauley

Thanks much David, I stumbled on this dxe-rtr-2 last week as well while I was googling around. Good to hear that somebody else in a similar situation has had a good experience with it.

David Salomon

YW.  Despite the additional cost, I decided on that instead of a front end saver type device because of the double protection (i.e. disconnecting on transmit as well as limiting the RF to a safe level).  I have tested the RF limiting protection by not connecting the keying line and transmitting, and it works well.  I don't recall the exact db level it allows through, but it's well below the point of damaging any SDR.  The other reason I use it is because I'm distributing the signal from my loop to 8 different devices.  I wanted to be absolutely certain I wouldn't **** them all up with a single mistake.

Martin AA6E

The radio is obviously "protected" since it hasn't burned itself out (yet).

One cheap and easy solution is to switch on the input attenuator or insert an additional attenuator in the Rx line, say 10 dB.  That goes against the grain, of course, but on the lower HF bands (probably 20 M too), the received signal levels are often high enough that you can do this without affecting the signal-to-noise ratio very much.

I'd say switching the power off on Tx is the best choice, however.  But you need to sequence it so that the DC goes off before the Tx RF goes on, and the DC comes on only when the RF has stopped, the same as you would do with a transmit/receive relay.

73 Martin AA6E

David Salomon

One more comment.  The advantage to an rf limiter solution vs a solution that either switches off the power or takes the loop out of line on transmit is that you can do SO2R.  You can just not connect the keying line on the RTR-1A and use only its rf limiting features.  That allows you to continue to use the RX-A antenna for receiving on slice A while transmitting on slice B.

In my setup, the LZ1AQ loop has built in overload protection, so the loop electronics are protected.  Realize, however, that an RF limiter will only protect the equipment after the limiter, i.e. the loop electronics at the loop end should be protected some other way.  I don't know if the Wellbrook loop has the same type of protection as the LZ1AQ loop.

Steve G1XOW

No protection on these preamp units and they are very vulnerable to TX power killing them. I have lost 2 of those with 400W from a TX vertical about 80 feet away. The designs from cross-country wireless are much more immune.