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Author Topic: Measuring resonant point of end fed dipole  (Read 13028 times)
KJ6QLD
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« on: February 20, 2012, 12:19:52 PM »

I've been doing some reading (here in particular - http://aa5tb.com/efha.html) about end-fed half wave dipoles.  In order to get some hands-on learning, I decided to experiment with one for 10 meters (being a convenient length for fooling with indoors).  So I cut a 10 meter half wave length of wire (cut a bit long at 17') and then cut a  counterpoise nominally at .05 wavelength, although I also cut it a bit long at about 20".  So I figured my half save section would be resonating somewhere a bit below 29 MHz.  I hooked up the half wave section to one pole of my antenna analyzer, and the 1/20 wave section to the other pole of the analyzer.  Between about 27 and 30 Mhz, my reactance was over 500 ohms (analyzer doesn't read any higher) and the resistance read zero (although this may also mean overlimit I think).  I did some snipping, 1/2" at a time, but nothing changed.

I would assume that I should get zero reactance at the resonant point of a dipole, no matter how far off center it is fed, and after doing some searching on the forums here I read some confirmations of that assumption.  I did get pretty good dip in reactance at around 21 Mhz, down to about 65 ohms.  So I'm puzzled.

Setting the whole end-fed with counterpoise idea aside for the moment, and just considering the total length of wire I was using (17' + 20" is about 19'), if I viewed it instead as an off center fed (but not end fed) dipole with a total length of 19' (fed 20" from the end), then resonance should be around 24-25 MHz.  So my readings don't seem to be making sense either way.

The last variable to consider is the analyzer itself - It is a VK5JST unit that I built myself, the first project I ever built, so maybe there's something haywire there?  It seems to do just fine on center fed dipoles though. 

Any one have a sage notion of something that I'm missing or doing wrong?

Thanks
Galen
KJ6QLD
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G3TXQ
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Posts: 1533




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« Reply #1 on: February 20, 2012, 01:03:30 PM »

Galen,

Strange things happen when you feed that close to the end of a "dipole". Treating it as a 19ft OCFD, EZNEC confirms that although there is a slight SWR dip around 25MHz, the reactance never falls below several hundred Ohms in that region. Of course the resistance is also very high at that feed position. You need to be at least 12%-15% away from the end of the dipole to get the sort of results you are expecting.

Your 21MHz resonance is very likely a common-mode issue. I don't know what length of coax you were measuring through, but the braid path could well look like a lower impedance route for current than your 20" counterpoise.

73,
Steve G3TXQ
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WB6BYU
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Posts: 13578




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« Reply #2 on: February 20, 2012, 01:43:53 PM »

Quote from: KJ6QLD

I would assume that I should get zero reactance at the resonant point of a dipole, no matter how far off center it is fed...



That's true - if you can find it.  The way most analyzers work requires that the impedance also
be within range before the reactance value makes any sense.  The impedance at the end of an
end-fed half wave is a few thousand ohms, and you won't get a good measurement.

(Not to mention that the resonant frequency is a function of the combination of the "antenna"
and "radial" wires, plus the affect of the analyzer case and any wires attached to it.  You can't
measure just the wire by itself without connecting the other side of the analyzer to something,
and that something will affect the reading.)

I'd use a dip meter for such measurements, but usually the wire just needs to be close
to resonance, and cutting it by formula is probably good enough.
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KJ6QLD
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« Reply #3 on: February 20, 2012, 02:20:47 PM »

Thanks - both answers helpful.  There would not have been a common mode issue, as I did not measure through any feedline, but connected the two radiator pieces directly to the terminals of the antenna analyzer.  Good to know about the limitations of antenna analyzers at very high impedances.

Since I don't have a dip meter, maybe the best way to determine resonance (if I actually wanted to verify by testing rather than just using the length formula) would be to cut the 1/2 wave section in half, adjust the length for resonance, and then put it back together. 
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WB6BYU
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« Reply #4 on: February 20, 2012, 03:04:10 PM »

You can get pretty close by attaching a second wire the same length to the analyzer and
tuning for resonance at half the frequency.  (But it doesn't work if you have loading coils
in the wire.)
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KJ6QLD
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Posts: 22




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« Reply #5 on: February 21, 2012, 09:22:44 PM »

Quote
You can get pretty close by attaching a second wire the same length to the analyzer and
tuning for resonance at half the frequency.  (But it doesn't work if you have loading coils
in the wire.)

I'm not sure that I understand why loading coils would prevent this from working.  I'm picturing a a radiator that is a resonant half-wave length at some unknown frequency (f), and the radiator is physically shortened by means of a loading coil.   You then used another identical mirror image coil loaded radiator and treat the two symmetrical radiators now as 1/4 wave elements of a center-fed half wave dipole resonating at half the unknown frequency (f/2).  If you can measure f/2 with an analyzer, then why wouldn't this work to find f for the end fed single loaded radiator?
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N3OX
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« Reply #6 on: February 21, 2012, 10:14:23 PM »

Thanks - both answers helpful.  There would not have been a common mode issue, as I did not measure through any feedline, but connected the two radiator pieces directly to the terminals of the antenna analyzer.  

You could still have  problems if you only had 20 inches of wire attached to one terminal.  The analyzer circuit board itself is probably a lower impedance counterpoise than 20 inches of wire but both are small capacitances.  

I don't think you can measure the impedances found at that insertion point either, but even if your analyzer could measure thousands of ohms of resistance and reactance, it is going to perturb your measurement.
« Last Edit: February 21, 2012, 10:16:26 PM by N3OX » Logged

73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
WB6BYU
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Posts: 13578




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« Reply #7 on: February 22, 2012, 12:23:24 AM »

Quote from: KJ6QLD

I'm not sure that I understand why loading coils would prevent this from working...



The problem is that the loading effect of a coil (or other forms of loading for that matter)
depends on the frequency and the relative location that the loading is added relative to
the standing wave on the antenna.  And it probably isn't the same at half the frequency.

For example, you need a much larger coil near the end of an antenna than near the
point of maximum current for the same shift in resonant frequency.  That's one of the
trade-offs with putting the coil higher up a mobile antenna - it raises the radiation
resistance, but it also requires a larger coil.  (It doesn't work to think of it as a certain
amount of inductance - or number of feet of wire in a coil - corresponds to a a fixed
amount of shortening.)  The higher the relative current in the coil, the more shortening
effect it has.

If you mirror two loaded dipoles as a single dipole on a lower frequency, the current
distribution changes:  the end near the new feedpoint now has a high current, while
the opposite end still has a low current.  A larger coil near the end would have much
more effect now because the current is higher.  In fact, leaving the inductance fixed
and moving it from the center of the wire to one end would raise the resonant
frequency of the original antenna (as it would have less effect) but would lower the
resonant frequency of the double antenna (because you are moving it close to the
feedpoint.)
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KJ6QLD
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Posts: 22




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« Reply #8 on: February 22, 2012, 11:19:12 PM »

Ok, after reading your response and doing some thinking, I believe I understand.  So if you did have coils, then you could instead measure for resonance with an analyzer by cutting the 1/2 wave radiator in half and measuring in the center (after which you could rejoin the halves for an end-fed configuration), correct?
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W0BTU
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« Reply #9 on: February 22, 2012, 11:48:18 PM »

"End fed dipole"?  There's no such thing.
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G3TXQ
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« Reply #10 on: February 23, 2012, 12:50:31 AM »

"End fed dipole"?  There's no such thing.

Isn't this still the IEEE definition of a dipole?
Quote
dipole antenna: any one of a class of antennas producing a radiation pattern approximating that of an elementary electric dipole.

NOTE - common usage considers the dipole antenna to be a metal radiating structure that supports a line current distribution similar to that of a thin straight wire so energized that the current has a node only at each end

Steve G3TXQ
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W5DXP
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« Reply #11 on: February 23, 2012, 06:10:17 AM »

"End fed dipole"?  There's no such thing.

We are not talking about fishin' "poles" here. Smiley Any antenna with two electrical poles (two standing-wave maximum voltage points) qualifies as a "dipole", by IEEE definition.

Strangely enough, a G5RV used on 20m doesn't meet the IEEE technical definition of a "dipole antenna" (as posted by Steve) and is actually a 1.5WL "quadrapole". Hence the multi-lobed radiation pattern. A 1WL vertical is a "tripole", not a monopole.
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WB6BYU
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« Reply #12 on: February 23, 2012, 06:54:53 AM »

Quote from: KJ6QLD
Ok, after reading your response and doing some thinking, I believe I understand.  So if you did have coils, then you could instead measure for resonance with an analyzer by cutting the 1/2 wave radiator in half and measuring in the center (after which you could rejoin the halves for an end-fed configuration), correct?


That's correct.  Technically you could measure it anywhere along the wire, but as you discovered,
if you meter handles a limited range of impedances you'll need to be somewhere in the center
half or so of the antenna to get a reading that is on scale.  Since most of my antennas are stranded
insulated wire I just grab it in a convenient spot and wrap a couple turns around the coil on my
dip meter - I presume the dip meter attachment for an analyzer would work in a similar fashion.
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KJ6QLD
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« Reply #13 on: February 23, 2012, 08:30:52 AM »

Got it.  Thanks for all the responses
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W0BTU
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« Reply #14 on: February 23, 2012, 11:18:40 AM »

Steve and Cecil,

I have always respected your technical ability, and have learned much from you. And when you post something on the forums, I generally accept it as accurate. But that definition of a dipole --an end fed antenna-- will certainly never be mine.

No less an antenna guru than Dr. John Kraus (W8JK) himself always drew a dipole as a straight linear conductor fed in the center, never at the end. (Once, for simplicity's sake, he left the balanced transmission line completely out of an illustration in his book Antennas, but it was still not fed at the end. It was assumed the feedline was present.) And I'm certain that he was by no means the only highly respected antenna authority who taught that.

I'm not going to take issue with someone who wants to call an unbalanced, off-center fed dipole a dipole (though in my mind, it's more of an end-fed antenna with a short radial). And if a straight conductor fed in the center is non-resonant, it is still a dipole. And of course, it can be called a dipole even if it isn't straight, such as an inverted-vee.

If I was in the business of manufacturing antennas, and advertised a dipole for sale, but my customers instead received a ground plane or any other kind of end-fed antenna whatsoever, I would rightly be accused of either gross incompetence or outright fraud.

I'm sorry, the idea than any end-fed antenna could possibly be labeled a dipole is not only confusing, but it's just plain wrong.
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