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[K8OCN]

I am thinking of getting another IC-7300 to put in my barn so I can use it for WSPR. The Barn is about 200 feet from my HF Vertical and The Windom Wire. I can put a wire antenna way back behind the barn say 200 feet. I am even playing with the idea of building one of the "On The Ground Loops" The barn rig would only receive.

On my station now I do run up to 1000 watts and about 50 watts when I run FT8 which I do a lot of. Because its a separate building I have no way to key a relay to ground out the receiver when I transmit. Does anyone know of any RF sensing circuit that I could use to ground out the receiver antenna when I was transmitting on HF?

Thanks for any help anyone can send my way,

Del K8OCN

[K6AER]

Most modern HF receivers will handle +20 dBm of signal with out permanent damage. Do note that anything much above -20dBm will be causing serious intermod in the receiver electronics.

You could just use a shorting relay that detects RF at a critical level on the receive port and shorts the receive input upon transmit from the other radio.

[KF6QEX]

Good place to start..
http://www.kk5jy.net/rf-clipper/

[K6JH]

If the rig is really receive only there are simple coaxial RX signal limiters available from DX Engineering, as well as inexpensive Chinese vendors on eBay. Similar to the KK5JY device KF6QEX mentioned. Or the fancier ones also have relays to short the RX antenna, via a PTT connection.

[KC0W]

 Spacial separation between antennas is obviously key. You should be OK at 50 or 100 Watts, just don't tune to an exact harmonic. Perhaps the worst combination to be running at the same time is 15/40 meters.   

 If you are running 1 kW, bandpass filters are a good solution. The more bands you operate on the more filters you will need..............An easy way of getting an immediate (theoretical) 20dB of attenuation is by using different polarities of antennas. A horizontal antenna (dipole) will be roughly 20dB down on a vertical antenna & vice versa. The orientation of the dipole to the vertical MATTERS. W8JI did some testing proving this............Coax stubs are another good solution & you might already have enough coax on hand to fabricate some HF stubs.

                                                                              Tom KH0/KC0W

           

[WB6BYU]

Would you have the receiver on in the
barn when transmitting from the house?

If not, just rig a relay to disconnect the
antenna from the receiver when the
receiver is turned off.  (If the radio
doesn't have a suitable passthrough
voltage, then feed it and the relay from
the same power supply and use that as
the ON/OFF switch.  (Or run it from an
outlet controlled by a light switch, so
when you leave the barn and turn off
the lights, it turns off the radio and the
relay disconnects the antenna.)

[AC6LA]

Coax stubs are another good solution & you might already have enough coax on hand to fabricate some HF stubs.       

This section of the AutoEZ documentation has a long-winded explanation on how to model stubs:

Stubs for Harmonic Suppression

Another good resource is from K9YC:

Some Q&A About Coax and Stubs for Your HF Station

Or just do a web search on "k9yc stubs".

Dan, AC6LA

[KC0W]

If not, just rig a relay to disconnect the
antenna from the receiver when the
receiver is turned off.   

 This is the most economical choice. I was looking at it from a contesting/SO2R perspective where two or more radios are being operated simultaneously. In his original question he states he wants to operate two radios at once. One dedicated to WSPR & the other for general operating.

I am thinking of getting another IC-7300 to put in my barn so I can use it for WSPR.

 The purchase of a IC-7300 solely dedicated to running WSPR is unnecessary but it's your money so do what you want. A used TS-50, FT-450 or any $400 radio will work just as good as a new $1,100 IC-7300. (if purchasing new is the route you plan on going)

 You might want to decide if you want to run WSPR on one dedicated band or WSPR on several bands. The easiest approach would be just to run it on one band and use a single coax stub or bandpass filter on the WSPR rig. This way it will be safe when you run 1kW on your other radio. WSPR is a very low power mode so I assume you will be running 5 Watts plus/minus. If this is the case you will be 99.999% safe from the WSPR radio damaging the front end of your other radio if even minimum antenna separation is used. That is to say I wouldn't worry about using any filters/stubs on your main HF rig if you operate WSPR at it's intended power level.

The barn rig would only receive.

 Why? You will be losing nearly all the benefits of WSPR if you only use it to receive.

         
       

[G3RZP]

Over 70 years ago, the  RMS Queen Mary (GBTT) ran full HF Duplex on AM with rx and tx frequencies in the same HF marine band - those  at 2, 4, 8, 12, 16 and 22 MHz. (The 6 MHz marine was too narrow to accommodate R/T) The separation between receiver room and transmitter room was around 400 feet: they had around 400 watts of carrier, suppressor grid modulated. So it can be done.....The receiver room was fully screened and leads to it filtered. Marine receivers for HF had to withstand 30 volts EMF of carrier on any frequency for 15 minutes without damage: Marconi Marine used a  germanium diode voltage doubler rectifier and a 2 transistor Schmitt trigger to drive a reed relay.

So the problem is by no means insurmountable.

[K5LXP]

Wonder and guess, or measure and know.  It would be an academic exercise to measure actual levels of coupled energy between the two antennas across the bands.  You may find no intervention is required at all, or if so, what degree or methods would work best.

Mark K5LXP
Albuquerque, NM

[W1VT]

A simple AD8307 based power meter is a cheap but effective way to make measurements.
https://www.qsl.net/df7tv/pm8307.html
Plenty of sources for boards that are made up using this chip.

[KH6AQ]

Does anyone know of any RF sensing circuit that I could use to ground out the receiver antenna when I was transmitting on HF?

On my station now I do run up to 1000 watts...
Del K8OCN

The MFJ-1708B RF Sensing TR Switch looks like it will do what you want. The product manual does not specify the RF power, but a check of the schematic shows it will switch on less than 1 volt of RF at the remote RX antenna and disconnect the receiver. This is 13 dBm.

Using EZNEC to determine the coupling from a 3.5 MHz vertical TX antenna to a 1/4 wavelength vertical RX antenna 200 feet away shows that 1 watt in the TX antenna produces 1 volt into 50 ohms at the RX antenna. This should trigger the TR switch. Transmitting at 1000 watts induces ~20 watts into the RX antenna and a TR switch like the MFJ-1708B is needed.

EZNEC
3.5 MHz
TX ant, resonant 1/4 wavelength vertical
TX ant, resonant 1/4 wavelength vertical terminated into 50 ohms
Each vertical elevated 1' with 4 x 68' counterpoise wires
Real, high accuracy GND. 0.005/13S

MFJ-1708B   https://mfjenterprises.com/products/mfj-1708b#product-manuals

[KH6AQ]

To sense RF on the MFJ-1708B ANT rather than the TX port the JP2 jumper can be placed between pins 2-3, the two right-most pins. If you decide to use an MFJ-1708B I will review this once again to make sure it is correct.

[WA3SKN]

For "receive only", a simple pair of silicone diodes across the antenna should work.  An old trick when separate transmitters and receivers were used.
However, I think a properly designed set of filters would be the most practical arrangement.  And you could simply go "QRP" and lower transmit power to solve the overload issue.

-Mike.

[W1RKW]

Can't use both rigs at the same time so how about a shorting antenna switch on both rigs? that's as simple as it gets.