High Q Mag Loop using panadapter

[gfmucci]

I will be soon receiving an MFJ TX/RX mag loop antenna.

I understand it has a VERY narrow band in both transmit and receive.

For those who have used this or similar narrow band loop, what has been your experience using that antenna with the SDR Console (or any) panadapter?

What are the good and less good operational characteristics?

[Max]

I will be soon receiving an MFJ TX/RX mag loop antenna.

I understand it has a VERY narrow band in both transmit and receive.

For those who have used this or similar narrow band loop, what has been your experience using that antenna with the SDR Console (or any) panadapter?

What are the good and less good operational characteristics?

Never owned one but read about these tuned loops in Pat Hawker's (G3VA) "Technical Topics" in RadCom over many years. He wrote very extensively about them. High Q with very small operational bandwidth, so my guess is you will almost certainly see exactly this on the panadapter, i.e. restricted bandwidth with sharpish peak. As the frequency of operation gets towards the lower end of the operational range the peak becomes very sharp indeed, with you being able to move maybe only a few kHz before you need to retune for SWR. On the plus side they act very much like a preselector or bandpass filter, and also in the same way work to reduce wideband local electrical noise.

Also, from a safety aspect, even at quite low TX powers, you want to be very careful as they generate very high voltages in the loop.

So generally, all good characteristics if working in restricted space or in an area with neighbours sensitive to larger antennas. I also understand that in the past they've been extensively used by the military. They often used rectangular loops (landscape orientation). I remember seeing one in action many years ago outside a military comms truck. Doubt very much if western military forces still us HF much these days!

For more info on magnetic TX loops generally see Simon's (G0ZEN) QRZ entry, bottom of page:
https://www.qrz.com/db/G0ZEN

Also maybe take a look at these. Pretty much covers all aspects of magnetic TX loop antennas:
https://youtu.be/19BBNiYZOy0

https://www.youtube.com/watch?v=XOGQyZe0ABA

73

Max

[gfmucci]

Nailed it regarding TX.

I understand that the behavior of mag loops that must be precisely tuned for tx have the same behavior on RX - they don't show much bandwidth of what is being received. There are broadband rx loops and narrow band rx loops. Does anyone have knowledge of how narrow band tx/rx mag loops look like in receive mode on panadapters? Do they do little justice to panadapters?

[KA1GJU]

Years ago, I briefly used my DIY 1m passive loop with a tuning cap on the larger of the two loops (looks like Icom AL-705). With its narrow bandwidth, open your SDR at max bandwidth on the desired band, and while watching the screen, turn the capacitor. You will see what looks like a wave, a narrow band of RF move through the scope. So now you have to tune the SDR to where you want to RX, then move the capacitor to move the antenna’s resonance to the desired frequency. Once that is done, the loop must be rotated as to not be trying to RX off the sharp nulls (broadside to the loop). As previously stated, I used it briefly. Just too many variables, moving parts, steps just to get a signal. Then, if you’re on 75m with a round table with multiple ops in all directions… good luck!


I have gone to long EFW’s in inverted L configurations on most of my SDRC Servers. Resonant antennas work fine for one band, but get rather deaf when used elsewhere. Since I have no control over where users listen, DC to Daylight, these 150’ EFW’s with a 9:1 unun at the feedpoint work well as and ‘all band antenna’. I still have two resonant antennas available, a Force 12 C3 beam, and a 160m inverted vee. Can be entertaining watching a user on the 10/15/20m beam while on 75 or 40m.

Unless you live in a 900 sq ft condo, I would spend your money elsewhere. Even then, you’ll be hearing everyone’s switching power supplies, medical equipment, plasma TV’s, LED lamps, touch lamps, UPS’s, cordless tool chargers, etc with that antenna. See my thread about the QRM I was getting from a bad computer power supply, at 2/10 mile away!

[gfmucci]

Thanks for sharing your experiences.

Yeah, I live in something like a 900 sq ft condo. No "wire" longer than 10' will be accommodated. Been thru all that, thus the loop purchase (MFJ-1786). I'm already using the MFJ-1708B SDR for 2 meter operation.

I have 2 (or more) options to view active frequencies:
1) Use a second, broadband, loop antenna on my RSPdx for RX, along with sufficient protection on antenna inputs since I don't have adequate space to sufficiently separate the TX and RX antennas. Any filter/suppressor suggestions?
2) Rely on Spot/Cluster apps to see where the action is within my selected band. Any app suggestions?

Any other suggestions along these lines will be appreciated.

[Max]

I have 2 (or more) options to view active frequencies:
1) Use a second, broadband, loop antenna on my RSPdx for RX, along with sufficient protection on antenna inputs since I don't have adequate space to sufficiently separate the TX and RX antennas. Any filter/suppressor suggestions?
2) Rely on Spot/Cluster apps to see where the action is within my selected band. Any app suggestions?

Assuming if you say condo that means other properties close by, all of which represent possible RFI sources, I'd be inclined to stick to the MFJ loop because of the previously mentioned advantages of (1) tuned magnetic loops seem to have more immunity for e-field RFI sources and (2) you have the "preselector" effect of narrow bandwidth.

You certainly need to be careful to protect the front end of any SDR Play receiver. By the company's own admission they are somewhat fragile in that department. Why not use the same MFJ SDR switch to isolate the receiver? Looks like exactly what you also need for HF. If it were me, though, I would not rely only on the RF switching. I'd be manually switching it using the PTT line from the TX just to be sure it never fails to go changeover.

If you do want to go the broadband RX antenna route, remember you not only need to think about protecting the SDR, you also need to think about the antenna preamp in the broadband loop, although I think I am right in saying that most of them have back to back diode protection already installed?

For cheap and easy diode ladder protection maybe take a look at this:

https://www.kk5jy.net/rf-clipper

Regarding Q2, why not use the cluster facility already present in SDR Console? It's very good! Just pick your preferred cluster server and away you go. I never turn mine off:

https://www.sdr-radio.com/dx-cluster

73

Max

[gfmucci]

Thanks, Max.

Yes, I am using the PTT input on the MFJ-1708 from the 13 pin out on the IC-7100. In fact I had a combo Hsend Vsend cable made up that I plugged into a little push button switch - button in for HF; another button in for VHF/UHF.

Thanks for your other 2 suggestions that I will look into.

I do have a couple of older Receiver Guard 2000 M's plugged into 2 RSPdx antenna inputs. Not sure how trustworthy they might be.

The Win4Icom rig control app that I use also has a Spot utility I'm currently trying to figure out. It uses Club Spot.net. Win's Spectrum Scope is more basic than ADR Consoles is, but good for a quick setup.

Off topic: Just received a message another solar flare is on the way.

[Max]

I do have a couple of older Receiver Guard 2000 M's plugged into 2 RSPdx antenna inputs. Not sure how trustworthy they might be.

The Win4Icom rig control app that I use also has a Spot utility I'm currently trying to figure out. It uses Club Spot.net. Win's Spectrum Scope is more basic than ADR Consoles is, but good for a quick setup.

Off topic: Just received a message another solar flare is on the way.

Link discussing the RG2000M here:
http://www.k8nd.com/Radio/CW_Skimmer/W8 ... -5000.pdf

Looks like it's probably fine if not cutting edge protection. I imagine if they were considered fine by the station manager of the large contester's group PJ2T (as per the PDF) then they must be OK but I'm no expert. I know that devices using multiple diodes as opposed to simple back to back pair prevent the non-linearity issues on receive from a simple diode pair. Again, with an indoor loop I doubt if this would be a major issue compared to a contester station's multi-element beams. The point he notes about static DC build up does not apply in your case with an indoor antenna I would not think.

Sorry, zero experience of the Win4Icom rig control app so will have to pass on that one, save to say the Cluster setup in SDRC is very straightforward. I would suggest giving it a go.

Thanks for heads-up on the flare. I think the big thing about the activity last week was multiple events (seven I think) from the same solar cluster of sunspots, but who knows. We will watch and wait!

73

Max

[Max]

You might also want to take a look at this thread, and the referenced ARRL document about safe distances for RF exposure versus power input.

https://www.reddit.com/r/amateurradio/c ... n_safetly/

Also just to reiterate the high voltages you may well generate on the loop @ 100W input. I think the main rule is "don't touch it when in TX"!

73

Max

[gfmucci]

Max,
Thanks for the additional info. I had a typo (oh really, not me) in my previous post. The SDR I am using is SDR Console, not ADR Console. But you probably gathered that. That is supposed to have a good Spots utility too, which I am in the process of figuring out.