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Author Topic: Antenna Analyzer for Open Wire Feeders  (Read 40325 times)
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K5UJ
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« Reply #50 on: September 25, 2011, 08:48:07 PM »

This might not be the right topic thread but I think in this one or maybe elsewhere, don posted something to the effect that if you lift your johnson matchbox so it is not bonded to a ground via the lug on the back panel, you achieve a balance of the currents in the open wire line.  I think I misread his suggestion--maybe he meant not grounding the inductor center with a center tap to ground.  In that case the MB is okay as is, for it has no center tap.  Well, I conducted a crude experiment here today.  with the MB cabinet strapped to a rod outside via a 3 or 4 foot run of 3 inch wide copper strap, I tuned it to 52j0 on 3885 and fed it 500 mw.  Then I went outside with a simple diode driven fs meter, one of those CB type things made by the millions with a telescoping antenna on it.  I measured the field about the feedline and noticed it was slightly stronger on one side.  I got away from the feedline and went to each end of the dipole standing on the ground under it and took readings.  The fields at the ends of the dipole were uneven; the one corresponding to the slightly stronger side of the feedline had a slightly stronger reading than the other.  on a scale of zero to 2 it was 1.5; the other end was 1.3

I repeated everything with the MB disconnected from the ground strap.  the feedline field was closer to uniform (not entirely but closer) and the ends of the dipole both measured 1.3 (I would have been happier with 1.5 hi hi). 

I have not yet given the floating match box full power--I gave it a modulated 50 watt test transmission to see if I could hear any signs of trouble like RF in the audio but I didn't notice anything different from the former state of things.  I'm still not sure if I want to try this as I have some sort of innate hesitation to run 300 watts with the grounded sides of the MB capacitors only grounded via the unbalanced feedline shield.  But, it does appear to result in a more balanced dipole and feedline. 

I'd be incomplete in describing the test if I did not also mention that one end of the dipole is held up by a tree; the other end is held up by a 50 foot aluminum tube that also serves as a 75 m. vertical (it has a 15 foot stinger on its top) with a ground system at its base.  It also holds up the 160 m. inverted L.  The ground system radials cover the back yard and extend up to the shack (a.k.a. house) where the MB ground rod is driven in at the open wire line entrance. 

For lack of any explanation I suspect the ground rod is somehow forming part of a counterpoise with the radials and mast and when the MB is connected to it, it works against it, functioning more like a slightly unbalanced off center fed dipole. 

Sorry if this is way off the topic, but now  that I think about it, putting a very weak signal into the antenna system and measuring the fields is a form of open wire analysis.  I wish I had a more sophisticated measuring instrument though.

I'll be surprised if anyone notices any signal strength difference with my matchbox ungrounded but I'll give it a try at night with 50 watts for starters.

Say, if I'm doing anything that might cause the MB to flash over with higher power or if for some other reason this is a dumb thing to do pipe up and warn me--field strenghts don't matter if something zorches.

rob
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« Reply #51 on: September 25, 2011, 10:31:15 PM »

Rob:
I have read (RSGB Handbook) that leaving the output side of the link antenna coupler is the way to go.  Grounding this side of the coupler (center of tank coil or rotor of split stator capacitor) increases the potential of the Marconi effect.    http://amfone.net/Amforum/index.php?topic=12469.5;wap2

Chuck
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k4kyv
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« Reply #52 on: September 25, 2011, 10:34:26 PM »

I am double checking to make sure my graph in reply #30 is correct.  Is it correct?

Looks like it is.



I have read (RSGB Handbook) that leaving the output side of the link antenna coupler is best kept floating.  Grounding this side of the coupler (center of tank coil or rotor of split stator capacitor) increases the potential of the Marconi effect.    http://amfone.net/Amforum/index.php?topic=12469.5;wap2

Too right!  Rob, according to my schematic of the 275 watt matchbox (not sure what size yours is, or if all sizes use exactly the same circuit), the coil is indeed floating, but the rotor of main split-stator capacitor C1 is grounded, and the two inner-most sections of differential capacitor C2 are tied together and grounded. That is effectively the same thing as grounding the mid-point of the coil. Try lifting the connection where the rotor of C1 connects to the sections at C2, from ground (which I assume to be the metal case of the Matchbox). I would also assume the ground terminal simply connects to the case. Leave the connection between C1 and C2.

The connection to C1/C2 and ground is necessary when the Matchbox is being used to feed a single-wire antenna, since that is the only way the circuit has a ground return. But when feeding a balanced two-wire line, the ground is unnecessary, and in fact, may cause unbalance at the feeders precisely for the same reason grounding the mid-point of the coil would: it is providing a ground return for common mode current and allowing the feedline/dipole combination to work like a vertical-Tee against ground in addition to working like a balanced parallel line fed dipole.  

Since the common-mode current is of the same polarity in both feeders, but the differential current is of opposite polarity in each feeder, the sum of the currents will be different in each, since in one feeder I (total) = I (common mode) + I (differential), and in the other feeder, I (total)=  I (common mode) − I (differential). In additional to the unbalance, the common mode current (probably much less than the normal differential current) nevertheless causes some feed line radiation, plus ground losses if the Matchbox is earthed to anything less than a good low-loss radial ground system.

Lifting the ground point at C1/C2 won't blow anything up, since this would not substantially change the voltages across the capacitor sections or at the band-switch connections. If anything, it might actually reduce them, since unwanted common-mode voltages/currents are not being added to to the desirable differential currents at one feeder. Lifting this ground will not affect the ground return at L1, the link. If you were using a link-coupled final and a balanced twisted-pair or parallel-line link instead of coax feed, there would be no need for this ground connection, either.

I would modify the Matchbox to have an external ground strap to C1/C2, that could be removed or added as needed.  It shouldn't be needed with a balanced open-wire transmission line. It is incomprehensible to me why Johnson didn't do it that way to begin with. I would keep the external ground connection to the metal case of the Matchbox, to keep the unit effectively shielded, and to prevent the box from becoming hot with RF, in case something goes wrong to unbalance the system, such as a conductive object contacting one of the feeders, or one leg of the dipole coming down.
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Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

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aa5wg
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« Reply #53 on: September 26, 2011, 12:34:18 PM »

Thanks Don.
Chuck
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« Reply #54 on: September 26, 2011, 05:01:47 PM »

Don, thanks,yes of course I see now.  I looked at the MB schematic again and it is obvious the v. across the caps should be less with them lifted off ground.  it all makes sense.  I have two KW MB so I'll do the cap mod on one and put it into service and perform my field strength measuring again and see what happens.
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« Reply #55 on: September 26, 2011, 05:46:50 PM »

I started a new post titled "Link Antenna Coupler Circuit Setup".  I have some more questions regarding link antenna coupler circuit selection.  Everyone's help is greatly appreciated!
Chuck
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