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Author Topic: Antenna Feedback  (Read 728 times)

Posts: 240

« on: February 16, 2003, 03:08:37 PM »

      A little while back I posted a problem I was having and recieved numerous responses as to what the problem was.  Now i need some advice on what to do about it.  Summary:  I have a wire loop antenna around the ceiling of my apartment.  I have it connected to the balanced wire terminals of my MFM-949E tuner.  When I transmit my SWR need responds correctly however my forward need goes to 100w even though I am only transmitting 25w - 30w.  The anomoly varies in severity by band.  If I use more than 40w the forward power need is pegged.  I have tried ferrite beads, 10 to be exact, on the cable between the rig and tuner with no results.  I have tried movie the radio away from the tuner with no results. Is this only going to stop when i get the tuner away from the antenna field?
Then I am out of luck as I can not put up an outside antenna on the owners orders.


Posts: 10248


« Reply #1 on: February 16, 2003, 06:15:19 PM »

What you have here is just too much RF. A VSWR bridge uses diodes as detectors, and if there is enough RF floating around (proximity to the antenna in this case), your reading will be suspect.

Incidentally, you should be feeding this loop with twin lead or other open type line, NOT coax. And almost no matter what you do, you're going to have problems with all of that RF floating around that close to your operating position.

Alan, KØBG


Posts: 3585

« Reply #2 on: February 16, 2003, 06:34:55 PM »

As I understand it, you have a coax jumper between the tuner and the rig, and you are either driving the loop off the balanced line terminals or using open wire line.

The problem is doubtless too much RF in the SWR meter circuitry. Now, since you are essentially inside the antenna you have "real RF," and then you have "near field radiation."  

My suspicion is you are getting enough near field to be the principal cause of the problem. After all, at least half of the RF you feed your antenna will wind up as near field radiation. And in your position the near field will be by far the strongest.

Now, your tuner won't be nearly as well shielded as your rig. If the rig's SWR meter works OK, I would log my tuner settings and then disconnect the tuner's SWR meter. You can touch up your tuner using the rig's SWR meter, and that's where low SWR counts anyway.

But if your rig's SWR meter is affected also the only alternatives I see are to either live with it or figure out a way to hide a more distant antenna. Or wait until the weather is better and find a more radio friendly QTH.

73  Pete Allen  AC5E

Posts: 1435

« Reply #3 on: February 16, 2003, 07:05:01 PM »

The only time I use a watt meter, forward needle?, is when I have to adjust my amplifier.  If the forward needle doesn't give you useful information, disconnect it?

Posts: 22


« Reply #4 on: February 16, 2003, 08:28:09 PM »

If your wattmeter is a true directional wattmeter and there is a mismatch between the antenna and the transmitter, the Forward position will always, and correctly, indicate power that appears to be more than the power output from the transmitter.  This is because the mismatch causes a reflection of power from the transmitter back to the antenna ADDING to the original forward power.  However, there is reflected power from the antenna, and this will appear as a reflection on the on the other needle, i.e. reflected power.  Subtracting the reflected power from the forward power shows you the net power transferred to the load -- this power will be the net power delivered to the antenna.  A properly adjusted directional RF power meter will always give you these indications -- with a seeming increase in the output of your transmitter.

An excellent treatise on this subject can be found in Walt Maxwell's book Reflections II -- where SWR meters, reflected power and wave theory applied to feedlines is examined and explained.


Posts: 42

« Reply #5 on: February 17, 2003, 08:35:03 AM »

You are flooding the metering circuitry with rf as described earlier, this is nothing new in the rf service world, how to fix it - dismantle the meter/tuner and use aluminum window screen over the meter face and any other unshielded openings, this may not be ideal operating conditions but you will never know the accuracy of the meter until you can keep the near field radiation away from it. If you have some patience you can ignore the meter if you twist the knobs very slowly until the rushing noise heard in your head set is as loud as possible, you have then fabricated the impedance the transciever is looking for at that frequency. If you other guys think I am kidding cover your meter face with a cardboard panel and twist em around, I learned this from a blind ham at field day many years ago and it works well in our club submarine station where you cant see most of the time because of the crowding observers in the small shack.
Hailing from NOARS Country,
73...Clark Beckman N8PZD
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