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Author Topic: MFJ-1786 will not tune reliably.  (Read 2130 times)
KB9TQN
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Posts: 12




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« on: November 17, 2012, 02:02:28 PM »

My brother gifted me with his old Yaesu 897D and an MFJ-1786 loop. I live in a basement apartment so conditions are less than ideal. I can't get access to the roof or anywhere outside because the condo association basically looks down on us like we're crapola. No big deal because my brother had the loop next to his desk with no problem getting out on a first floor apartment. If I got out even a little bit I'd be happy.

Everything is set up and ready to go. For a day the antenna tunes up ok. Not great, SWR is 1.5:1 but I'm running ten watts for JT65 on 20 meters. I bounce around bands and have different results on tuning up but I figure it's no big deal. An antenna can't be perfect on everything. The next day was different. More often than not the antenna wouldn't get below 2:1. Sometimes it would be 2.5:1 or higher. Different bands had different results but no luck on any. If I stepped away for a while and came back things would be better and I'd have fun but things have been hit or miss for the last week. I swapped coax, moved the antenna throughout the house to avoid coupling, glared at the antenna, opened it and the control box up to look for anything grossly out of whack. I tried shaping the coupling loop without success.

This afternoon I broke out the old matchbox my brother gave and cut a dipole for the center of 20 meters. After I got the cats to stop chewing on things and halted their races through the house I flipped the radio on and tuned up. After working the matchbox for a minute I got the SWR down to 1:1. I increased power. 1:1. I increased power to the max 100 watts. 1:1. So the radio is good. A dipole is good. The loop is bad.

The question is how and why? Is it just coupling? Wouldn't the dipole do the same? If not what could be wrong with the loop? What can I look for?
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STAYVERTICAL
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Posts: 864




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« Reply #1 on: November 17, 2012, 03:26:10 PM »

Hi,

As an MFJ1788 user I suspect your problem is coupling.
I have found that if I do not get the loop in the clear, it is hard to get good swr figures.
It does not need to be high off the ground (I am assuming you are using it vertically), but must be away from conductive materials.

I have never had to adjust my feedloop and found that if I used it inside, I had similar problems.
Once taken outside (away from metallic structures), it played 1:1 SWR on all its designed bands (40m to 15m in my case).

Remember, the MFJ178X magloops are very high Q tuned circuits, basically old style metal detectors - and will be influenced by nearby conductors.

Another quirk I have noticed with the MFJ magloop is that its capacitor/motor drive system really needs a super-slow-motion setting.
You may think you have gotten to the bottom of the swr curve, but I am betting that if you give the up/down button a quick press you can get lower.
It may not be true in your case, but with me it is certainly true, and I instinctively do that while operating.
When using high power (100W), or as the loop heats in the Sun, it will also change tuning slightly - needing a small correction.
All of this fiddling is simply because the loop is high Q which means low loss construction, and that comes with a bandwidth penalty.

As for using a tuner with a wire antenna, that is a different story.

First, some background:

Any bit of wire when fed with radio frequency energy will lose some as radiation into space.
This can be represented as a mythical resistor absorbing this amount of energy.
This mythical resistor is called the radiation resistance of the bit of wire for radio energy at that frequency.

For a magloop the radiation resistance is very low, in the order of tenths of an ohm or less.
To get useful power "absorbed" by this radiation resistance (radiated) we need large currents in the order of tens of Amperes.
The small magloop does this by being built with low loss techniques in the loop conductor and tuning capacitor.
This ensures that at resonance the current will be as large as possibe - but only at resonance.
Since a high Q signifies a low bandwidth, the antenna is low bandwidth.
This is why we need a tuning capacitor to cover a wide range.

With a dipole it forms a standing wave of current and voltage along its length which also results in resonance.
The resonance in this case is however due to the length/diameter of the wires, and its nearby environment/ground.
The Q of the resulting structure is lower and so the bandwidth is wider.
This means it will work over a wider bandwidth before you reach high SWR points.
The radiation resistance of a dipole is also around 70 ohms, so you only need small currents to give useful radiation.

Both antenna's will radiate, but they are solutions to different problems.
A dipole is a quick, easy and well behaved solution - keep it if you are happy with it.
The magloop is a restricted antenna environment, multiband solution - if you can possibly put it outside it should work well.
I use one every day with good results.

One little suggestion - be aware the magloop will have up to 8000 volts of radio frequency energy at the top when using 100W.
Even 5w will be around 1000 volts.
Do not allow anyone to touch the magloop when transmitting (may be hard with your cats!).
Also, stay away from the magloop when transmitting, it has a very intense field nearby - prudence suggests caution.
Keep people at least 15 to 25 feet away when using over 5W.

Good luck,

73 - Rob
« Last Edit: November 17, 2012, 03:31:41 PM by STAYVERTICAL » Logged
VE7TIT
Member

Posts: 62




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« Reply #2 on: November 18, 2012, 08:31:28 PM »

...I suspect your problem is coupling.
I have found that if I do not get the loop in the clear, it is hard to get good swr figures.
It does not need to be high off the ground (I am assuming you are using it vertically), but must be away from conductive materials.

Another quirk I have noticed with the MFJ magloop is that its capacitor/motor drive system really needs a super-slow-motion setting.
You may think you have gotten to the bottom of the swr curve, but I am betting that if you give the up/down button a quick press you can get lower.

I concur on both points.
Point 1, coupling: A small magnetic loop seems to couple into its immediate environment very closely. If you have a lot of wiring, metal ducting, or (my case) big cast iron radiators nearby, it won't tune up anything like it will in the comparative freespace of a suburban back yard.

Point 2, tuning: The biggest surprise for me in fooling around with mag loops has been how amazingly touchy the tuning is, especially at the bottom of its frequency range. My homebrew 31" diameter loop goes into and out of tune on 20 meters over a capacitor movement of 2/3 of a millimeter.

One more thought: if you're running 100 watts while you're sitting close to the antenna, you're running too much power.

Good luck, have fun, and I hope you get to move into a more ham-friendly place soon. Smiley
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NH7L
Member

Posts: 31




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« Reply #3 on: November 20, 2012, 12:59:54 AM »

I concur with the other responses you've gotten. I used an MFJ-1788 for a couple of years inside an apartment in a steel-reinforced high-rise concrete building. I was never able to position the loop more than 2 to 3 feet away from steel. As a result, I could never tune an SWR suitable for transmit, and so I used the antenna strictly for receiving,  placed just inside a window.

Taking the same antenna outdoors, or into a wood-frame two-story house, I was able to transmit with it very well.

You may find you can get a better SWR by experimenting with the antenna's location and orientation. It's very directional when the loop is vertical like a bicycle wheel but is omnidirectional when placed horizontally like a hula hoop.
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