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If you have a coil wound on a form and pass a piece of ferrite material in the center, reactance increases. If you pass a piece of non-ferrous material (Aluminum) through the center, it kills reactance. Do I understand this right?
Total posts : 45366Yes you do.
There is a tool in a radio tech’s toolbox called a Magic Wand. It is a plastic rod and on one end is ferrite and the other is brass. When the ferrite is brought near an inductor in a tuned circuit the resonance shifts lower in frequency. The brass end shifts it higher. This is used to tell whether you need more L or less L to get resonance at a particular frequency.
The brass probably kills the Q.
Neil
Total posts : 45366Thanks, Neil. Just wanted to make sure that I was still thinkin’ right.
I was looking at some of my older posts from back in ’06 and they got me all fired up again. (My quest for a remotely tuned base loaded Part-15 legal AM antenna) I started to experiment with an idea back then but didn’t follow through with it.. 😉
MICRO 1700’s thread got me thinkin’..
Total posts : 45366A remote antenna coil tuner would be a nice project and I have kicked around some ideas which have not come to fruition. Most of them involve changing the inductance of the coil by mechanical means such as by moving a series “trim” coil closer or away from the main coil or rotating a small series coil such as is done with a varometer. The coil in use here now ( http://s6.postimage.org/e72kzpzdt/AM_Loading_Coil_0179.jpg ) has a small movable coil on the main coil form which allows very precise adjustment but it is a by hand adjustment.
One obstacle for a remotely tuned coil would be the actuating mechanism. Metal objects such as a small motor will affect the coil losses. Lead wires can affect the stray capacitance. Maybe a pneumatic system could be built using a piston or diaphragm to move the coil. From experience it doesn’t take much movement of the “trim” coil to affect tuning.
Perhaps a moving coil such as in a loudspeaker could be coupled with a plastic rod to move the tuning coil.
An air coil with light gauge wire will contract with applied DC current. Maybe a carefully RF decoupled variable DC supply could tune this by changing the current.
It may be that a nearby ferrite rod could be used but it might affect the Q and coil losses negatively.
These are some brainstorming ideas to ponder.
Neil
Total posts : 45366In my avatar, you see the coil that I build via instructions from the SSTran site for the outdoor antenna. I don’t remember the pipe size at this moment but I know a beer can fits the inside diameter..
The coil has too many windings for any “good” resonance so that’s why they have the multiple taps. I connected my AMT3000 to the bottom winding at the very end of the wire using the whole coil. Using a CB field strength meter, I slid a beer can into the coil at the bottom and found a peak easily. Might add a little more resistance to the coil but I don’t believe that it’s enough to worry about..
I’m thinkin’ I can use some fiberglass fish sticks inside of some PEX pipe as a guide. Install the fish sticks through the PEX, put a beer can on the antenna end of the stick and fill the inside of the can with some insulation foam to keep it on there. Position it below the coil and Vala! Shove the fish stick in and out and the can will move up and down inside of the coil. Position the field strength meter near the antenna and use some field glasses to see it.. (Could possibly tune a ground that way too!)
Going one step farther, I have a powered automobile antenna. I can use that as the top trimmer and adjust the antenna length from a distance..
The only issue is the antenna trimmer on the board of the SSTran. I contacted Phil about using a varactor diode circuit but he says the induced voltages wouldn’t allow it..
Thoughts?
Total posts : 45366This is a very interesting subject
which I have thought about also –
this remote tuning idea.If my 38 turn “ferrite” tuned coil
hadn’t been sitting in the backyard
in the mud for three years I would
have thought about it more. But
now that I have found this coil
again, I’m really wondering how and
why it worked. I mean, why did it
work at all? We may never be able
to answer this question. We
may be able to figure out the properties
of the coil, but we don’t know the
properties of the ferrite rod.And also – why did it work SO WELL if
the Q was diminished by the ferrite? I
was getting very good range out of
this set-up.By the way, I had the ferrite bar on
the end of a long plastic rod, so
I could sort of stand away from the
coil when I tuned it. When I was
done tuning, the rod just stuck out
the open end of the coil. I had placed
something under the rod to hold it up
in the air so the whole thing wouldn’t
fall out of the end of the coil onto
the ground. The ferrite was just laying
in the coil. There was no support structure.
And the coil was on it’s side. The coil wasn’t
part of the antenna – it was part of the
AMT-3000s internal circuitry – so it could
just sit outside the transmitter on it’s
side. The SS-Tran and the coil were sitting
on top of a small plastic crate, so they
would be insulated from the ground. I think.I really should keep better records.
To add humor to the thing – although this
is not a joke – it is really true – the
set-up was inside a small children’s
plastic play house.Gosh, now I want to repeat the experiment.
Maybe I should drink some beer and put
a beer can into the apparatus, as well.Seriously.
Bruce, DRS2
Total posts : 45366There’s only one company that I’m aware of that has given any thought to a remote ATU. They understand the pitfalls of being near the antenna system and tryin’ to get a good, accurate peak. I spent many hours tweekin’ and peekin’ and watchin’ things change when I backed away from the antenna with my screwdriver in hand. It frustrated me big time watchin’ my peak change with every move that I made. (That’s where the beer came in handy!)
Total posts : 45366There was an article in QST magazine where a guy put up a 160 meter short vertical antenna.
His remote tuner coil was made from 1/4 inch copper tubing. The diameter of the coil was about 12 inches. The large size was his attempt for a Hi-Q coil.
One end of the coil is secured, the other end attached to a hinged plate. The plate is actuated by a motor driven screw jack.
This allowed him to compress or expand the coil to tune the system.
He seemed to think it worked quite well.
Total posts : 45366Your description of the beer can tuning is intriguing. There are two explanations I can think of for this. The non-ferrous material lowers the inductance and/or the capacitance added turn to turn affects the coil. The concern is that the eddy currents in the aluminum would suck energy from the signal. But if it works, why not?
Maybe, as you suggested, it would be easier to adjust the length of the radiator using the antenna motor you mentioned. I don’t know how sensitive the tuning is for length but if a difference of a few inches would work then you could start just shy of 3 meters and go from there.
You mentioned frustration in approaching and backing away changing things. It sure does! When I did my field test I used a current transformer/voltage pickup and a digital scope to monitor the phase angle between V and I at the feed point with resonance being 0 degrees shift. Moving my hand from about 6 inches to 1 foot away changed the angle by more than 45 degrees. (This turned out to be a good way to tell which side of resonance it was tuned and whether more or fewer turns were needed.) Some remote tuning scheme would eliminate this effect as well as making it easy to compensate for other things that detune the antenna such as snow, etc.
Neil
Total posts : 45366The discussion continued about insulating the coils so they wouldn’t short out against each other when compressed. I bought two of them at the time and still have them! I might mess with them some day too..
As for the eddy currents, wouldn’t they be sucked-up by just about any material used for an adjustment medium? I figure that ferrite reacts to the magnetic field and redistributes it back into the coil and aluminum just kills the reaction, nullifying the reactance, and making the coil just plain resistive. Perhaps there’s a gain with using ferrite? (Beer cans are easier for hillbillies to find..)
I measured the FM antenna on my truck. It measured out to 32″. I’ll figure the electric antenna extends at least that far. I need to get that thing out and measure the bottom part to see what the total length is..
The SSTran base loaded antenna only has 8″ of adjustment on the radiator. With the base of the electric antenna just above center, it may give some added capacitance to the radiator. And with ~32″ of adjustment! Just gotta keep it to a legal length when fully deployed. I know when I moved mine an inch or two it made a difference on the peak. Talk about fine tuning! I don’t believe that the wiring for the power to the electric antenna would mess things up much as long as they are kept stable. (Maybe run ‘um inside of the lower part of the radiator?) I believe that there would be enough tuning capability to work around it if it did. Or, add some chokes to the wires to decouple them..?
*Patent Pending*
Total posts : 45366To answer my speculation about varying the radiator length I did a quick experiment by attaching my antenna probe to the feedpoint of the loading coil and adjusting the radiator length by +/- 2 inches. The result is:
Length,,,,,Phase,,,,,Current
+2 in,,,,,,-66 deg,,,,,,38 ma
+1 in,,,,,,-37 deg,,,,,,49 ma
+0 in,,,,,,,,,0 deg,,,,,,57 ma
-1 in,,,,,,+27 deg,,,,,,55 ma
-2 in,,,,,,+50 deg,,,,,,47 maSorry about the commas but without them the table doesn’t align. Changing the radiator length up and down from 3 meters has a large effect on resonance. A small change detunes the system quite a bit. From theory, a 45 degree shift in phase is the -3dB point which means half power point in a resonant circuit.
From this it appears that a remote tuning system which varies the radiator length a few inches either side of nominal resonance length would be adequate for fine trimming adjustments.
Neil
Total posts : 45366Your data is very interesting, Neil.
I had wondered about this very thing
for a long time.Bruce, DRS2
Total posts : 45366I’ve been pondering another thought.. Gonna need an engineer on this one..
I’m thinkin’ on the line of a gamma match, like one used as a matching device on a yagi antenna. Basically two metal tubes, one inside of the other, insulated away from each other by whatever, at a close tolerance. Have a calculated coil on the bottom at the feedpoint. (Bottom of the outer tube) Adjust the inner tube up and down to find resonance. The inner tube would be the bulk of the length of the antenna. No coil taps and it would be real easy to figure out a remote tuning device..
Capacitive coupling at the base to the radiator. Adjustable too! If the coil was close, it would be easy to tune..
Comments?
Total posts : 45366Ever see ONE OF THESE or SOME OF THESE?
Well your beer can variable inductor loading coil is EXACTLY what kind of effect those linked piston compression capacitors have, changing value by adjustment of a plate or plates in relation to the other plate or plates. In your case, a round metal object slid in or out of the loading coil making it change inductance.
Ever mess around with an AM radio’s internal ferrite bar antenna and moved the coil around? Same thing.
Resourceful, not new but resourceful. Reminds me of hearing the stories from my grandfather who served in WWII finding whatever they could to set up field communications. Most here would be quite surprised at how things got done back in the day of the punch card and typewriter.
RFB
Total posts : 45366New way to use it! I’m just tryin’ to spark some imagination and see where it takes us.
Guess I’m tryin’ to design without bein’ real technicial. Just keepin’ it basic and simple. The theory comes after the fun! Simply make it work and then wonder why..
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