T/R Relay Isolation

[flintstone mop]

Hello AMers

I need to use my 40M vertical as a 'AUX antenna" for the MFJ noise canceller. That will be the AUX antenna the canceller needs to compare to the "TX" antenna to get rid of some buzzy power lines that have been bothering me lately. They buzz on 160 sometimes, or it goes to 40M or both. It doesn't seem to be an automatic night light. This pops up during the day time and mostly when it is damp.

Guys, I know I need to get out there with the MFJ noise finder and start some troubleshooting, while the line is acting up. Everything is becoming a challenge lately for me...

The AUX antenna must hear the same buzz as the TX antenna to compare properly and be able to reduce the noise down to nothing. 10 over buzz to an S-3 is a nice change.

The question is: Will a T/R relay be enough isolation from the AUX antenna not to couple QRO power into the MFJ noise canceller?

One day I found out how much power was coupled back to the AUX port and it blew out a little lamp that protected the "front end" of that circuit. 50 watts seemed to be the power that came back through that coupling.
YES,,,the vertical is close to the transmit antenna...Can't move it.

Thoughts or suggestions?? Another "AUX antenna"??
Thank you

[W1ITT]

Without taking actual measurements, everything is just an opinionated guess.  Every installation is different.  I would run a swept two-port isolation measurement with my network analyzer if it were my situation.  I use mine to put Cheerios in the bowl so it just happens to be handy. but I realize that not everyone has one.  So we must move to Plan B.
If you were to use some extra coaxial relays, controlled by the transmit-receive switching, you could (a) add extra series isolation for the receive phaser front end, or (b) use a relay to terminate the phaser front end with a 50 ohm load during transmit, or (c) both of the above.
Without good swept isolation measurements, the safest thing to do is get crude and throw the whole kitchen at the problem.

[AB2EZ]

Fred

Your post raises a number of questions, particularly if your 40m vertical is as tightly coupled to your main transmitting antenna as your post implies.

In normal use (when you are not using the vertical antenna as an auxiliary receiving/noise antenna)... is the coaxial feedline entering your radio room: open?, shorted?, terminated in a 50 ohm dummy load?

When you made the measurement of the power being coupled from your transmitting antenna into the vertical antenna, was the coaxial feedline from the vertical antenna driving a 50 ohm dummy load?

A coupled power of 50W into the vertical antenna seems unreasonably high. If it is that high, then the termination across the coaxial feedline from the vertical antenna would have a significant effect on the radiation pattern of the transmitted RF output signal, as well as the SWR looking into the main antenna feed-line. I.e. the vertical antenna would act as a significant parasitic element of the main antenna system.

If the coupling is a lot less than your post implies, then a simple approach for using the vertical antenna as a noise cancelling antenna might be to place a resistive attenuator between the incoming coaxial feedline from the vertical antenna, and the MFJ noise canceller's noise antenna input. The magnitude of the impedance looking into the MFJ's noise cancelling input is probably a lot higher than 50 ohms. A suitable input atenuator might be a 1000 ohm resistor in series with a 50 ohm resistor, across the output of the coaxial feed line... leading from the vertical antenna... with the input of the MFJ box taken across the 50 ohm resistor. I.e. the noise antenna input, when listening to received signals, does not have to be the full output of the vertical antenna. You can verify this by inserting a resistive attenuator in the path between the vertical antenna and the MFJ box, and using the MFJ box the cancel out the noise. The MFJ box allows you to attenuate the main antenna input... to compensate for the attenuation of the noise antenna input. As an extra measure of protection for the MFJ box, you can place a pair of plain vanilla 1N4148 pin diodes (in parallel and in opposite directions) across the input to the MFJ box. Alternatively, an isolating relay... as you originally proposed... can also work... if it is sequenced properly.

Stu

[flintstone mop]

Thanks for the replies.
Stu:

 It is on a sequencer and on the Antenna port. The vertical is directly under the dipole and there may be 30 feet separation. That is why there are watts...The reading was taken with the vertical terminated into 50 ohms and a bird wattmeter.

W1ITT:

The idea of terminating the T/R relay when it switches into a little dummy load, would be a good idea.

How accurate the watt reading is an interesting thought, as it is just induced 160M RF into the vertical that is not resonant for 160M. No tuners or coils at the base of the vertical. Full 1/4 wave long for use on the entire 40M band.

[AB2EZ]

Fred

How much power were you launching into the coaxial cable leading to the dipole when you measured 50W, into a 50 ohm dummy load, coming back down the cable leading from the vertical antenna?

Stu

[flintstone mop]

Fred

How much power were you launching into the coaxial cable leading to the dipole when you measured 50W, into a 50 ohm dummy load, coming back down the cable leading from the vertical antenna?

Stu

The dipole is full length for 160M, 65 feet high, fed by OWL. The power was QRO AM 1.880 mhz. The vertical is directly under the dipole.

[AB2EZ]

Fred

Okay. With that much power coupled into the vertical (when the associated coaxial feedline is terminated in a 50 ohm dummy load)... I suggest that you leave the vertical antenna feedline unterminated (open), and that you use a small ad-hoc noise antenna. You probably can "make-do" with a noise antenna that is less than 10 feet long.

I have the Timewave equivalent of the MFJ noise canceller... and when I need to use it (typically on 40m), I use it with a 4ft vertical antenna (no feedline) located in the shack.

Stu

[WBear2GCR]

A couple of reversed but in parallel diodes across the input/front end of the MFJ? No?

That and an additional shunt relay to ground at the input of the MFJ ought to be pretty bullet proof,
and the relay does not need to be anything very large or special, I expect.

                     

[flintstone mop]

Fred

Okay. With that much power coupled into the vertical (when the associated coaxial feedline is terminated in a 50 ohm dummy load)... I suggest that you leave the vertical antenna feedline unterminated (open), and that you use a small ad-hoc noise antenna. You probably can "make-do" with a noise antenna that is less than 10 feet long.

I have the Timewave equivalent of the MFJ noise canceller... and when I need to use it (typically on 40m), I use it with a 4ft vertical antenna (no feedline) located in the shack.

Stu

OK Stu.
Located in the shack threw me way off. I thought the AUX antenna has to hear the same offending noise as the Main antenna for the canceller to be effective?  The noise in the house is not the offender, and the electrical line noise is not heard in the house with the MFJ's little whip that comes with the unit. Maybe I'm making more work for myself with the AUX antenna outside....................dunno

The BUZZ seems to drift around and affect different bands. Sometimes 160 sometimes 40 or both. It's amazing how the box works and nulls out noise.

Here's my crazy money spending idea....A vertical antenna closer to the house and away from the TX antenna. CB whip with a stud to screw into a mag mount base. Mag mount base on some big ferrous plate = ground plane. MFJ antenna pre-selector with some preamp to "Tune" AUX vertical to the band where the noise is and shoot that into the AUX in of the canceller. The antenna IN of the preselector would NOW need the protection and isolation that the AUX of the canceller had. I can even have the DC power to the pre-selector cut off during TX.

Overkill ?? or turning something simple into something stupidly expensive??

[AB2EZ]

Fred

If the combination of: 1) where the noise is coming from, and 2) the relatively long wavelength (160m)... makes a short noise antenna in the shack ineffective... even with the MFJ's noise antenna gain adjust and the MFJs main antenna gain adjust available to equalize the noise signals coming from the main antenna and the noise antenna... then I suggest that you go back to your original plan.

A) Use a properly sequenced antenna relay to disconnect the vertical antenna feed line before you transmit on the main antenna feed line. Do not terminate the vertical antenna feedline with a 50 ohm dummy load when transmitting into the main antenna (unless you want 50W of power to flowing into the dummy load). If you normally leave the vertical antenna feed line open (e.g. disconnected) when not in use, then leave it open when transmitting. If you normally have the vertical antenna feedline shorted (e.g. by a manual antenna switch) when not in use, then short it when transmitting. That way, the new use of the vertical antenna as a noise antenna, when receiving, will have no effect on your normal, main antenna radiation pattern, when transmitting.

B) Between the auxiliary antenna relay and the noise antenna input to the MFJ box, place an attenuator consisting of a (first) 1000 ohm series resistor and a (second) 1000 ohm resistor from the MFJ side of the first 1000 ohm resistor to ground.

C) For extra protection of the MFJ box's noise antenna input, place a pair of pin diodes (e.g. 1N4148), both in parallel with the second 1000 ohm resistor, oriented in opposite directions, to serve as a bi-directional peak limiter.

If that works... you're all set.

Stu

[W1ITT]

AB2EZ...
 Please see my earlier post. The suggestion was not to terminate the noise antenna into 50 ohms during transmit.  It was to terminate the input of the phaser following an isolation relay.  This provides additional assurance that the receiver chain is not floating at some high impedance and therefore subject to damage.  This is fairly common procedure in some military applications with which I have dealt.  The EME lads often do it to protect their expensive low-noise preamps as well.  If you already have some T-R switching sequenced and operating, it's cheap insurance.
Norm

[AB2EZ]

Norm

Agreed!

Looking at the schematic of the MFJ-1026, which is in the manual that can be downloaded here:

http://www.mfjenterprises.com/Product.php?productid=mfj-1026

It appears that the "auxiliary" input (a.k.a. noise antenna input) already includes a 100 ohm termination (in parallel with the adjustable input gain pot) and (if not using the whip antenna) already includes a pair of back to back (in parallel) pin diodes.

Therefore, a properly sequenced antenna relay between the vertical antenna feedline and the MFJ auxiliary input should be all that is needed. I.e. no need for the additional pair of external 1000 ohm resistors, and no need for the additional pair of PIN diodes.

Note that the whip antenna is only connected to the noise antenna path if the internal preamp is activated from the front panel. This may be why the whip antenna did not produce the desired results, when used as a noise antenna.

Stu

[flintstone mop]

Thanks Guys
I'm seeing the responses here and will go with what has been suggested. Making perfect sense in my head.

Thank you

[flintstone mop]

OK Guys
Here's a snip from my post several steps up::::

Here's my crazy money spending idea....A vertical antenna closer to the house and away from the TX antenna. CB whip with a stud to screw into a mag mount base. Mag mount base on some big ferrous plate = ground plane. MFJ antenna pre-selector with some preamp to "Tune" AUX vertical to the band where the noise is and shoot that into the AUX in of the canceller. The antenna IN of the preselector would NOW need the protection and isolation that the AUX of the canceller had. I can even have the DC power to the pre-selector cut off during TX.

All of the toys arrived today in the mail... collapsible 18 foot vertical whip with 3/8 inch stud to go into the mag mount, the MFJ Active Antenna Pre-Selector.
The pre-selector tunes up noise on all bands and I'll interrupt the DC power to it, in Transmit. Its front end will be on the T/R relay, which is unterminated and about 100 feet away from the big TX dipole.
I'll have to arrange the desk for noise tuning to operate now. I think I'll be ok, for protecting the goodies.
Hopefully no mishaps operating this system, until I can get a handle on the power lines.


 

[AB2EZ]

Fred

Okay... we all know that you just wanted to buy a bunch of new toys.

If the original noise source goes away... you can buy a new noise generator to replace it... and continue to use all of the other new stuff to cancel it out.

Have fun!
Stu

[flintstone mop]

Fred

Okay... we all know that you just wanted to buy a bunch of new toys.

If the original noise source goes away... you can buy a new noise generator to replace it... and continue to use all of the other new stuff to cancel it out.

Have fun!
Stu

thanks Stu for and the others on how to protect the extra antenna and system. I was happy that the toys worked to be able to hear the same noises and buzzes that come around in Ham radio

[Opcom]

Those are some pretty good ideas. In my case its a broadband amplified loop, but the same issues so moving it farther from the dipole should help me too.

[KD6VXI]

I've got a 1025 phasing box,  same thing sans 1 preamp and antenna input (board is screened and ready for the parts,  so same same).   

The guy I bought it from stated he had snipped the internal lamp and jumped it.   His reasoning was he didn't like the light flashing inside while he was txing.   This threw a red flag immediately,  but it was PayPal,  he stated it worked and guaranteed it,  so I bought it.

Great box.

My point is,  the back to back diodes internally do a good job of protecting the first jfet.   I've read stories of that jfet being blown,  but it sure would take a lot.

Make sure you hook up the internal TX switch as well.   One wire and it will TX / RX switch.   I believe W8JI website has the schematic he recommends for protection on the aux ant port.   If not there,  I'll try to dig through the cache on my browser.

OK,  dug through the browser.   It wasn't JI.  http://charlessocci.com/2013/02/18/a-protective-tr-relay-circuit-for-the-mfj-1025-and-mfj-1026-noise-canceller/  ....   There's the circuit on that page,  tried and true.

But,  Tom has some interesting addendum to help with bc band use,  help IMD,  etc.

Hope it helps.   I like mine,  a lot.

--Shane
KD6VXI