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Measuring Feed-point Impedance

Measuring Antenna Feed-point Impedance

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: I am a new ham home brewing antennae with my MFJ259b. I’m reading and slowly understanding The ARRL Antenna Book and ARRL Handbook. I recently read the classic article “My Tunes My Antenna!” by Byron Goodman (QST Mar 1956, pp 49-51, 124; also QST, Apr 1977, pp40-42; also QST Nov 1991, pp 33-35 ARRL Members link http://www.arrl.org/members-only/tis/info/pdf/9111033.pdf.).

I have two questions: How do I reliably measure an antenna’s feed point impedance given I must always use at least some length of and I cannot assure a precise match between the feed-point impedance of the antenna and the of my transmission line? How do I reliably measure the resonant frequency of my antennae with my MFJ259b?

Mr. Goodman describes what the characteristic impedance of transmission line really means. He defines it for a theoretical lossless line. “The characteristic impedance of a transmission line is the value of resistance that, when used as a termination for the line, makes the input impedance of the line independent of the of the line.”

So a transmission line with 50 Ohm characteristic impedance (coax) with a load of 50 Ohm resistance at one end (antenna) will always present a 50 Ohm resistive impedance at the other end of the line () no matter its length.

But, if the load (feed point impedance of the antenna) does not match the characteristic impedance of the transmission line then the measured impedance at the other end depends on the electrical length of the line. The electrical length of the line changes with frequency of the signal as well as the physical length of the line.

Ok, so in the real world I want to measure the changing impedance in a 20 meter vertical dipole (or a 2 element Quad or whatever) as I place it at different heights. I read that resonant vertical dipoles have a feed-point impedance of around 100 Ohm (ARRL Handbook 2007 Fig. 22-1 of Vertical Half-Wave Dipole at 0 Height). The problem is that I don’t have transmission line with a characteristic impedance of 100 Ohm. For a given frequency, the measured impedance (and SWR) will vary with the length of the transmission line.

Mr. Goodman says that the impedance will repeat each ½ wavelength along the transmission line. It will be all resistive at multiples of ¼ wavelength and match the load at multiple of ½ wavelength.

So I conclude -- I haven’t yet read and learned about Smith charts.-- that I MUST use a transmission line length (in my case coax) that is multiple of ½ wavelength in order to “easily” and accurately measure the feed-point impedance of my antenna. (That means use the transmission line velocity factor to calculate what a ½ wavelength multiple would be and use it before hooking up the MFJ259b.) If I don't do that, then I cannot measure the feed-point impedance of the antenna directly from the antenna analyzer except where it matches exactly the characteristic impedance of the transmission line. Correct?

It seems to me that trying to avoid the use of a transmatch or is more work than just using one and not caring about feed-point impedance of an antenna.

Thanks for reading through everything.

Tom McDermott: Hi,

There are several options:

http://www.eham.net/ehamforum/smf/index.php?topic=39250.0;wap2[1/6/2015 8:39:29 PM] Measuring Antenna Feed-point Impedance

1) Put the analyser right at the feedpoint of the antenna (sometimes a bit tricky).

2) Find a length of cable that is exactly 1/2 wavelength long (or multiple) at the single frequency you want to measure. Connect that between the antenna feedpoint and your MFJ. Then measure the impedance.

3) Find the exact length in degrees, of whatever feedline you are permanently using at some frequency near where you want to operate. You will have to disconnect it from the antenna to make this measurement. You can then calculate the length in degrees for nearby frequencies with reasonable accuracy.

The Smith Chart is your friend. The impedance of will be about 52 ohms, depending on the exact cable. Just assume it's 50 ohms and has no loss. A reasonable assumption for good feedlines, HF bands, and shorter feedlines.

Plot your measured R+jX point on the Smith chart, then rotate the reading around the center of the Smith chart by the length in degrees of the feedline at that frequency (the degrees are plotted on the chart periphery).

A google search on Smith chart tutorial will provide you with plenty of material.

-- Tom, N5EG

Tom Rauch: When measuring the resonant frequency and impedance of antennas (antennae are on insects), you either have to normalize out the effects of the feedline, measure right at the feedpoint, or have a nearly lossless line that is electrically 1/2 wave long or an exact multiple of 1/2 wave at the measurement frequency.

Those are the only three choices.

Now here is the tough part. The line will only be correct at a certain specific frequency and exact harmonics of that frequency. This makes it tough to sweep a line input over a wide frequency range and find the real load characteristics on various frequencies.

When I test antennas, I connect them to a known length line that is an exact multiple of 1/2 wave at the desired frequency and use a 259B on that frequency or I use a network analyzer that normalizes out feedline characteristics so it effectively measures the termination on any frequency.

When I wrote the manual for the 259B years ago, I did my best to warn people about misusing some of the measurements. This includes resonance and loss measurements. I hope those warnings are still in the manual, because at one point someone edited the manual and messed up the distance to fault and other measurements.

73 Tom

Steve Katz: Reading one reference is good, but reading a dozen is better.

Do learn about Smith charts, and you can download them free from several web sources.

In practice, it's not important to measure the impedance of your antenna, which is a good thing because it's very difficult to do for the reasons you stated. It's much easier to model it using computer software, which is 100% accurate (probably more accurate than making measurements with inexpensive equipment) provided you know all the elements involved.

Download EZNEC demo version, which is pretty powerful. It will probably entice you to buy the licensed version, which is more complete and not expensive compared with the $10,000 worth of lab equipment it easily replaces.

http://www.eham.net/ehamforum/smf/index.php?topic=39250.0;wap2[1/6/2015 8:39:29 PM] Measuring Antenna Feed-point Impedance

It will show you impedance change vs. frequency, vs. height above , vs. length, vs. proximity to other materials, vs. wire gauge(element diameter), vs. pretty much everything.

Your 1/2-wave dipole is very unlikely to have a Z of 100 Ohms at any height you'd be using.

WB2WIK/6

Dan: The feed impedance repeats every half wavelength, but it also is transformed in a calculable way for any line length.

There's a program called TLW that comes with the ARRL antenna book that will calculate this.

I use VK1OD's transmission line calculator a lot, and it also has this capability.

You can choose a transmission line type and length and frequency of operation. Then select "Zin" as the type of mismatch and enter it in R±jX form. Click "Calculate" and a new window opens that includes the calculated "Zload," which is the load end at of the transmission line.

I usually use it the other way around, I enter "Zload" and it calculates the impedance at the source end of the transmission line and the loss in the line... but it works both ways.

------

Overall though, in a practical sense, you don't care about the *precise* antenna impedance. You care about these things:

1) You care about line loss. Very bad mismatches increase the loss in the line significantly (especially with coax, less so with ladder line), so using the transmatch and letting 'er rip can cause you to lose a lot of signal. To know if this is going to happen or not, though, you only need to be in the same ballpark on the antenna's feed impedance. However, mismatches that are bad enough to cause significant line losses can't really be measured accurately with an MFJ-259B anyway. It only measures 7<|Z|<650 ohms and is more accurate the closer you get to |Z| = 50 ohms. I find antenna modeling software to be indispensable in calculating matching networks for high and low impedance antennas, then when I've got the approximate components to get me within a hundred ohms of the correct resistance and reactance, I can use the MFJ- 259B to tweak both down to 50+j0 by adjusting components. I could never use the '259B to actually measure the feed impedance of my 60 foot tall vertical on 40m, it's too high, but I can use it to see which of my L network components I need to adjust to match the antenna to coax.

2) Once you've calculated, guessed, estimated or measured the approximate antenna impedance and have confidence that your chosen transmission line won't be too lossy, you can "tune" the antenna with the feedline and just hunt for 50 ohm SWR that will keep your rig happy... or you can trim the antenna slightly and look for the same.

In the case of tuning an HF dipole fed with modest lengths of 50 ohm coax, it is always as simple as putting the antenna up at the desired height cut a little bit long, and trimming it until the SWR gets low at the end of the coax. You can do this confidently without worrying about feedline losses, because once you bring the SWR down at the shack end you've ensured that the antenna is getting close to being matched to the line, and the whole system is nearing a flat 50 ohms.

It's a little dangerous to do this with very long lengths of coax or at VHF because the line loss can be high enough with a mismatch to cover over the fact that you haven't really managed to get your antenna matched to the line. If you have 75 feet of RG-58 on 146MHz and you short circuit or open circuit it, the SWR at the input end will be just under 2:1, good enough for most casual builders... of course, you've just built yourself a coax dummy load... no power radiated at all, really.

http://www.eham.net/ehamforum/smf/index.php?topic=39250.0;wap2[1/6/2015 8:39:29 PM] Measuring Antenna Feed-point Impedance

A while back I stuck a connector and 75:50 ohm on one end of a 500 foot roll of RG-6 quad shield just to see what would happen. I had good SWR all the way down to 10MHz ;-)

But if you're talking 100 feet or less of okay quality coax on 20m, it's hard to misinterpret the SWR readings at the radio end in a way that you'll stop trimming the antenna before it's really tuned right.

73, dan

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