Tuning a MFJ Loop with a Flex

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I'm in a high rise condo. I've tried various wire antenna, including a homebrew trapped dipole.

I decided to try the MFJ 1788 (40-15) Mag Loop. It dramatically is out performing my trapped dipole.

However the reason for the post is with the display on the Flex, tuning it is simple. The first time I tuned I follow the instructions. However on the display, there is a significan "bump" in the receive signal, making it very easy to tune, visually.

I'm trying to post a video to youtube and link it, but....

Basically if you last freq was say 40m, and you switched to 20m, pan the display out very wide so you can see from 40 to 20. If the antenna is still down at 40, you'll see a big spike in the receive signal where it's tuned. Hit the up button on the antenna controller (the fast button) and watch the display scroll up, 'till it gets close to your freq.

Then move the display back in to a much narrow range and then fine tune the antenna with the slow buttons. With a little practice it really get very easy.

I was very nervous about the loop, as I'd watched all the tuning examples on youtube, and it seem troublesome to change frequencies, much less bands. But with the huge flex display, just throw the MFJ manual technique out and tune visually.

Below is a static shot showing on 20m what the peak receive signal looks like (until I can get a video posted). Just imagine this zoom out and then watching that peak scroll down the screen to where you are going to operate.

Brent W8XG


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Brent Parker

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Posted 3 years ago

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Jim Gilliam

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Very entertaining and interesting post. Thank you!


Jim, K6QE

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SteveM

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Brent,

I've considered buying a mag-loop to play with as an alternative to my vertical. Conventional wisdom says that mag-loops reject noise when compared to wire antennas. However, it doesn't look promising when I see S7 noise on 20m. Is it really that bad with that antenna?

(Edited)
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Brent Parker

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Steve,

That first static shot, the antenna was in my shack, a few feet from my computers, with a lot of noise. They were painting my balcony, so the antenna was moved in.

It's back outside now. Here is a new shot on 15m. Background noise between signals is about an S4. Signals are well above the noise. This is Sunday during the DX contest! I've been working europe all morning.

In this shot, the antenna is under 2:1 from 21.208 to 21.379 and is centered about 21.300.

So, while the background comes up vs the dipole, the signal comes up more.

Hope that helps.  Thanks,

Brent W8XG
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Steve - KD8QWT

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Mag loops are pretty cool indeed.  They make for great SWL since you can tune right into the station.  Be careful with the SWR.  Sometimes a visual peak may not be the best tune point for the lowest SWR.  Tuning becomes even more interesting when the bandwidth is very narrow, although the Flex will do a nice job of visually representing where the tune point is with a sharp needle point spike.  Fun stuff!
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Jim Gilliam

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I use the loop as a secondary antenna on the MFJ noise eliminator. I find when I point the loop to the offending noise source, the noise to signal ratio is much higher than that of my regular dipole antenna. I am able to get 25-30 dB noise rejection and it brings my received signal into perfect clarity.


Jim, K6QE

(Edited)
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Barry-W4TGA

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Great info Brent.  I do essentially the same thing when using my truck mounted Tarheel antenna and operating from my 5th wheel trailer.  I use the vendor supplied rocker switch to move the antenna up or down and tune for max received signal on the band I want to operate.  Works great!
73,
Barry, W4TGA
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Walt

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Loops are great to use.  But both a blessing ( they fit almost everywhere) and a curse if you want to see the entire 40 meter spectrum for signals - the loop is just too narrow.

I have three of them - a Welbrook loop for rx only, an MFJ for the boat, and an alexloop for the portable rig.  Really like them all.

Cheers
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Steve - KD8QWT

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They do tend to get too narrow for SSB, but digital modes can extend their useful range.  I'd never be able to do SSB on 80m, but WSPR works just fine.  Probably psk31 as well.  The challenge would be to tune to the proper offset.  I do often hear stuff on 160m, even though my tuning range is from just the other side of 80m to just past 30m.  I can hear better at 20m than a simple 20m dipole at 15', sometimes.  It really depends on noise levels.  If you can tune them remotely to fine enough resolution, they can work pretty well and still be camouflaged.  I'm contemplating a second loop for 20m through maybe 17m or 15m. 
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Dan -- KC4GO

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I have been using an MFJ-1788 for 40 Meter SSB almost daily as  RX only for over a year with great results. Don't find the bandwidth limiting. Great SNR improvement. I have it on RX-A on the 6500 with a vertical ANT-1 and a long wire w/SGC tuner on ANT-2. Not good for looking at full band this is true and slow to tune. 
http://www.mfjenterprises.com/Product.php?productid=MFJ-1788

My unit covers 6.940 to 23.670 MHz and takes about 40 seconds to tune from end to end. 
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Don-KB6TSQ

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Brent,

I use this method for off air tuning of my dipoles.  I set the differential capacitance knob to a value used by the band and change the inductance by utilizing a roller inductor.  As mentioned earlier, the lowest SWR is usually achieved on either side of the peak.

Thanks for sharing
73
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Jim Gilliam

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A good ball park setting is just to watch the noise floor rise to its highest level.
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Scott - WM7C

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I did a video a while back. https://youtu.be/NKS0RrX27VE
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Burt Fisher

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I wish there was great narration on the video that fully explained the process
(Edited)
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Lee, Elmer

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@Burt

lemme see ... push the up or down button and watch the resonance move up or down on the panadapter.  When the resonance is over the freq of interest take your finger off the button.   Glad to be of help.

Thanks Scott and Brent for the interesting video/article.  I may have to get one of these.

73  W9OY
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Burt Fisher

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Take microphone attached to video camera, speak into it as the wild action happens, post on You Tube
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Peter K1PGV, Elmer

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I've been fascinated by mag loops ever since I worked a ham in Germany on 20 meters PSK who was running low power into a mag loop on his dining room table.


I should give one of these a try. Thanks for the fun post. Brent.


Peter

K1PGV
(Edited)
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Brent Parker

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Peter,

It's on my balcony today, which is about 85'. I've got it mounted vertical with a main lobe to the NE. The DX contest is on. This morning I've worked a SK3, SN2, KC0, PJ2, so europe and South America are working well. It was a mix of 20m and 15m.

So far, it's out performing my trapped dipole. The reading out there on the net, this should equal a full sized dipole. As my dipole is about 1/2 size, it seems to be better.

As I have a significant antenna issue in the high rise, it seems to be a good solution. I'm also thinking, for a quick field application, it will be very easy and quick to set up, vs stringing up long wire antennas.

There was a comment earlier about the background noise in my posted picture. My shot was to show the tuning. The antenna was in my shack, about 4' from all my computers, horiziontally mounted. They were painting my balcony. So, the noise (from all my electronice) was a S7.

Now that it's back on the balcony the background is more about an S3-4. However it's really the separation of the signal from the noise that counts. My trapped dipole is "quiter", but the separation on a given signal from the noise, on the dipole might be 12db, but the loop separation will jump to 18-20db, on the same signal.

Really the purpose of the post is the quick visual tuning technique, with a flex. I've got a legacy radio on my desk, and tuning the loop, changing freq's is a pain!  On the flex, it's just very easy. It's the seeing of the signal, that just makes a huge difference.

I've been reading about these for awhile, but looking at all the youtup videos on tuning I was hesitant. I finally pickup up on. As I said earlier, the first tune was "by the book", then I discovered I could see the tuning resonant freq in the display, and quickly developed my own, much faster technique.

The only drawback is I have to be in the shack to tune the antenna. Guess I need to create an interface to push the up/down buttons remotly!

Thanks,   Brent W8XG
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Lee, Elmer

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http://iw7dmh.jimdo.com/remoting-ideas-1/remote-morse-key/

Enzo created a version of this that will switch between 2 output pins by changing a box on the remote program  

73  W9OY
(Edited)
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Lee, Elmer

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One other thing DDUTIL has a macro that will set the pan bandwidth with one button press 

DD6PBW0:.035;  sets the pan bandwidth to 35khz   so you could have 2 buttons one that would set BW to some very wide value and one that would set BW to some more normal value.  In addition DDUTIL has a toggle function that would allow you to toggle these 2 macros 

73  W9OY
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Brent Parker

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Lee,

I had not idea the DDUTIL could do that. I just implemented it. F1 goes back to narrow and F2 goes to very wide for a quick tune/band change.

I had to read the DDUtil manual a bit to get the syntax right but it only took a moment.

Thank you for the tip..................very useful.

Brent W8XG
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Lee, Elmer

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It turns out there is a toggle function as well so you can toggle 2 different macro memories

DDTG01:069:070;  will toggle the contents of macro slot 69 and macro slot 70  

there is a pan + and a pan -  DD6PZM+;  DD6PZM-;  pressing these will act like a +- on the pan

there is a pan center DD6PBW0:.019;DD6PCF0;  PBW sets the band width of slice 0 and PCF centers the freq flag in the bandwidth of slice 0  this is a dual macro but you can write it as 2 singles  All of these would be quite useful for your MFJ application

73  W9OY