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Author Topic: Open wire antenna resonances  (Read 2695 times)

Posts: 3657

« on: March 20, 2012, 11:49:08 AM »

This has me baffled, but I am sure I am missing something obvious Smiley

I was outside playing with my 110 foot open wire diopole, using my antenna analyzer and
an AEA CIA-HF analyzer to see how well my Johnson Matchbox would work, and whether
I needed to adjust the open wire length.

I can "tune" the antenna system to resonance on the desired bands, but I am also seeing
fixed resonant frequencies that I can match, but not move to a higher or lower frequency.
What causes these resonant points, and can't they moved by adjusting the Matchbox?


Posts: 1169

« Reply #1 on: March 20, 2012, 02:18:46 PM »

More of a situation resulting from the ladder line length, lots of sweetspots not requiring a tuner close to 50 ohms on various bands but very narrow.

Posts: 1790

« Reply #2 on: March 21, 2012, 08:34:06 PM »


I am not completely clear on your question(s), but it appears there may be a misunderstanding of what a Tuner (Johnson Matchbox, in this case) actually does.

Specifically: An "antenna tuner" does NOT change or do anything with the antenna or transmission line. "They are what they are on any given frequency."
(in terms of size, resonance and impedance presented at the end of the transmission line to the tuner). Obviously, the values of impedance, resistance, etc. that
the Tuner "sees" will vary by frequency. That said, all that the Tuner does is create an impedance MATCH between the "load" appearing at the end of the open wire (ladderline, etc.), and the 50 ohm coax going to the rig.) The tuner is NOT "tuning the antenna to resonance": it is just creating a match that allows power to be transferred. Obviously, there will be some actual resonant points as you sweep through the spectrum, but, again, those are are function of the physical length/properties of the Antenna and transmission line SYSTEM. Again, the tuner does not CHANGE them or "make" resonance.... it just "offsets" reactances, etc. to create a MATCH between the two transmission lines.  Again, the tuner does NOT change anything external to the tuner.

I guess another analogy is that the Tuner "fools" the rig into thinking it is seeing a low reactance 50 ohm load that it opposed to what is REALLY going on out on the open wire or ladderline which is a high SWR, high impedance, highly reactive "load" that the fixed 50 ohm out put of the rig would "hate" and not work with.

The Johnson Matchboxes have a limited range of impedance/reactance loads they will create a match with. The small MB will "typically" create a match between
about 40 and 2,000 ohms.  The KW box will create a match with "loads" between about 45 and 1,200 ohms of Z.  These numbers may vary some depending upon
what type of complex load the box sees and the frequency.

My main antenna is like yours: it is a 128 ft. Center Fed system fed with 450 ladderline to a KW Match Box.  I was able to find a line length that allowed the Match Box
to create a match on all bands but 30 M and the upper part of 80/75. To get it to work on 30 M I had to either add or subtract line, but that would "mess up" the other bands, so I ended up just putting a small shunt inductor across the tuner terminals and it has worked beautifully. Adding external C or L to the tuner is a way to "cheat" and extend the range in some cases. You can also put an UNUN on the INPUT of the tuner and extended its matching range too in some cases.

We have so many HF bands now that is probably almost impossible to find a combination of antenna and line length that will present a "reasonable" load to the tuner
on ALL bands. That is just cold reality. You are going to have to do some picking and choosing in terms of bands and line length. Beyond that you will have to
add or subtract transmission line or add a lumped constant external to the tuner to modify the load presented to the tuner.

Bottomline:  the Antenna is resonant at the appropriate length and related harmonics and the combination of the line will modify that. The Tuner will NOT *change*
                 those conditions. It just creates a match to allow power transfer between the two different lines. (hopefully, with reasonable efficiency). The range
                 that the tuner will create a match is limited by the size of the inductors and capacitors in the tuner.

73,  K0ZN

Posts: 4311


« Reply #3 on: March 22, 2012, 06:20:20 AM »

This has me baffled, but I am sure I am missing something obvious Smiley

Maybe here's what you are missing:

The open-wire feedline is a series-section transformer whose length determines the resonant frequency. By proper selection of open-wire feedline length, one can resonate a 75m dipole antenna system on any HF frequency thus eliminating a conventional tuner.

73, Cecil,

Posts: 3657

« Reply #4 on: March 22, 2012, 07:52:50 AM »

I should have been clearer.... for example, on 40 meters, using
the CIA-HF, I can get a good one-to-to match using the Matchbox,
I can see the SWR improve at a particular impove, by adjusting
the coil's resonant point and using the differential caps to achieve
the best impedance transformation. I was also seeing matches at
nearby frequencies that weren't affected by the resonating cap
in the Matchbox, although the differential cap would improve the
SWR when adjusted. On some bands I had double dips on the
screen that were close in frequency that were confusing.

Cecil, I'm going to modify the Matchbox so I can tune it from
the shack.  I'd like to stay with coax coming into
the shack since it is much easier to deal with the common
window grounding system; and having the Matchbox outside
eliminates about 60 feet of open wire. In fact, I think I bought
one of the geared motors from you several years ago!

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