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I recently built a 340 MHz dipole for test purposes. My NanoVNA-H4 was calibrated initially at the transmitter end of the coax cable. The first attachment shows the response of the antenna with transmission line from 200 MHz through 500 MHz. It was quite surprising to see the result. The transmission line is 15 feet of RG-58A/U coax.

The next step was to calibrate the NanoVNA-H4 at the antenna end of the transmission line. As you can see in the second attachment, the difference is quite dramatic.

I'd be interested in any comments. There is a 24 MHz difference between the various peaks and valleys in the first chart. It turns out that the transmission line is a half wavelength at 24 MHz after accounting for the velocity factor. It appears that frequencies that are multiples of 24 MHz come into play. I wonder if there is also an effect from frequencies that are odd multiples of a quarter wavelength.

73, Kent
AA6P

Without a balun, the coax shield is also part of the antenna.
Got any Ferrite beads?? ?Big ones that you can slide over the coax, or?better yet some of the clip on ones often used on AC power cords?
Clip one or two of those on the coax near the antenna and I bet both plots will be very similar.
Kent

On 7/20/21 5:53 PM, KENT BRITAIN wrote:

Without a balun, the coax shield is also part of the antenna.
Got any Ferrite beads?? ?Big ones that you can slide over the coax, or?better yet some of the clip on ones often used on AC power cords?
Clip one or two of those on the coax near the antenna and I bet both plots will be very similar.

Also, are you physically next to the antenna when measuring at the antenna? At 350MHz you are a really big interfering structure.

-kb, another Kent.

Thanks all for the comments.

I did try three ferrites at the antenna end of the transmission line and did not see any change. Two were the clamp on type. I believe they were designed for frequencies above HF but I don't know the ferrite mix. The third ferrite was a mix 31 FT-240 core with several turns of the coax wound around the core.

I was far enough away from the antenna in both cases to not see any effect on the readings. Calibrating at the end of the coax is also done because you don't want the NanoVNA in close proximity to the antenna.

QST published a very informative article in November 1991 called "My Feed Line Tunes My Antenna!". The author was Byron Goodman W1DX.

I believe the SWR and Phase plots in the first attachment may be entirely normal and explained by transmission line effects since the antenna impedance is not 50 ohms. Note that the frequency scan was relatively wide. These kinds of effects might not be noticed to the same degree with HF antennas.

73, Kent
AA6P

I'm curious if you first calibrated the transmission line with a 50 ohm load at the far end. Does the transmission line come away from the antenna perpendicular to the elements? A half-wave dipole impedance will start at low values near the ground (unless you have a LOT of money, a 160 meter dipole would not show 73 ohms), then tend to wiggle around a 73 ohm value as ultimately achieved in true free space. Are you attempting to adjust the impedance that the antenna is showing to the RG-58 characteristic impedance? Check to make sure it is 58 and not 59!

Just a few thoughts...
K0AM

That W1DX article appeared in QST three times, November 1991, April 1977, and March 1956. There is a lot of misinformation about transmission lines/SWR in the amateur community and I suspect ARRL feels it is worthwhile to republish the article once in a while. There are several excellent books about the subject available that can clear things up. I highly recommend the ARRL publication "Another Look At Reflections" by the late Walter Maxwell, W2DU. Although he puts on the afterburners once in a while, his information appears to be correct - and a real eye opener for someone who has misinterpreted transmission line theory.

K0AM

Frank - Thanks for the comments and information. The calibration for the first attachment was the standard Open, Short, and 50 ohm Load calibration done at the Channel 0 SMA connector on the NanoVNA. The first attachment would show the antenna with transmission line as seen by a transmitter or receiver.

The calibration for the second attachment used the same Open, Short, and 50 ohm Load calibration but at the antenna end of the transmission line. That effectively removes the transmission line from all measurements to show the characteristics of the antenna itself. That is one of the more powerful features of a NanoVNA.

The two legs of the dipole were bent down slightly to improve the match. Technically it is an Inverted V Dipole. The feed line does come straight down from the feed point.

The coax is Berk-Tek RG-58A/U low loss foam coax. The characteristic impedance is 50 ohms.

I believe the first attachment shows the resonant frequency of the antenna just ahead of the low point in SWR. Compare that low with the second attachment.

73, Kent
AA6P

Certainly, the feedline can act as an impedance transformer - but it's also a phase shift. In particular, it will change the phase of the reflected wave. Let's say I have an antenna that is resonant (X=0) but is a slight reflector (S11 mag = -20dB). If I had the VNA at the feedpoint, the SWR would be low, and the phase of S11 would be zero. Now, let's put in 50 degrees of feedline. Now, the S11 phase will be 100 degrees - 50 degrees on the way out, the reflection at zero phase, then 50 degrees more on the way back.

So that's why you want to calibrate at the end of the feedline - it also accounts for the loss in the feedline so you can measure the true SWR at the antenna. While everyone *says* "the SWR is the same every where on the feedline" that's not true for a lossy transmission line. The SWR gets better, the farther from the mismatch you are. Go out and get 100 feet of RG-174 and put any antenna at the far end for 2.4GHz. With 60 dB of loss, any antenna will have an awesome VSWR at the transmitter end. It's the MaxComm matching unit all over again.

Is it possible that your coax is not really RG-58A/U?

I created an EZNEC model of a free space 340 MHz inverted vee. The element lengths and angle between them were optimized for a 50+j0 ohm feedpoint impedance at 340 MHz. The complex feedpoint impedance between 200 and 500 KHz was then calculated over 101 steps and exported to SimSmith.

In SimSmith, 15 ft of various types of coax were added. The results are interesting. (This could have been done in EZNEC, but I find it easier to use SimSmith for this type of thing.)

With 15 ft of 75-ohm RG-59, the FREQ vs SWR plot looks almost identical to yours. The lowest SWR is at 352 MHz which is very close to the 353 MHz point you observed. My plot is attached.

Here's a quick and easy way to tell whether your coax is 50 ohm or 75 ohm. Calibrate the NanoVNA at the port as you originally did. Now connect the antenna with your 15 ft of coax. Sweep from 200 to 500 MHz. Now display the Smith chart instead of the SWR plot. If overall, the plot circles are roughly centered on 75 ohms, the coax is 75 ohms. (The 75-ohm point will be to the right of the chart center by about 10% of the chart width.) If the plot circles are roughly centered on 50 ohms (at the center of the chart), the coax is 50 ohms. Two example plots are attached.

When you calibrate at the end of the coax, you remove its effect, and you'll get the same results whether you calibrated with 50 ohm or 75 ohm coax. The SWR plot will then look like the one you got just as it did with my modeled version. Plot attached.

73, Dave
NU8A

Attached are some useful info for making VHF and UHF reference dipoies including a neat design for an impedance matching balun using semi-ridgid coax.

Thanks so much Jim, Dave, and Ron for your replies and all the information. I really appreciate the modeling and all the documents. I will need some time to review and understand all this information.

The coax is marked RG-58A/U on the outer jacket. It was purchased 40 years ago but is in perfect condition and has never been used outside. It is a smaller diameter than RG-8X or RG-59 and requires a thicker reducer when used with a PL-259 connector. I am actually using a UG-176 reducer but with some RG-8X outer jacket placed over the smaller coax.

The Smith Chart circles are centered on 50 ohms. I tried a 25 foot length of RG-8X and the Smith Chart circles were also centered on 50 ohms. However, with the longer feed line the frequency difference between the various peaks was about 15 MHz rather than 24 MHz.

I'll check the Smith Chart one more time with the RG-58A/U.

I attached an image of the adapter used for the calibration at the end of the coax cable. It may not be ideal, but it appears to work over a reasonable frequency range and it also allows use of the SMA calibration standards supplied with each NanoVNA.

73, Kent
AA6P

Dave - I had one other thought. Would differences in Velocity Factor explain the different results?

The velocity factor can vary quite a bit across different types of coax. I really noticed that when using the TLDetails program from AC6LA software. There is also a big difference between solid dielectric and foam.

A different velocity factor would change the electrical length of the transmission line. That would change the frequency differences between the various peaks and valleys.

I believe my cable is 14 feet 10 inches plus another 6 inches for the SMA to SO-239 adapter.

73, Kent
AA6P

On 7/22/21 2:48 PM, Kent AA6P wrote:

Dave - I had one other thought. Would differences in Velocity Factor explain the different results?

The velocity factor can vary quite a bit across different types of coax. I really noticed that when using the TLDetails program from AC6LA software. There is also a big difference between solid dielectric and foam.

A different velocity factor would change the electrical length of the transmission line. That would change the frequency differences between the various peaks and valleys.

I believe my cable is 14 feet 10 inches plus another 6 inches for the SMA to SO-239 adapter.

Sure.. what phase shift do you measure with the far end open at your frequency of interest.. Divide by 2, and that's your cable length (electrically)

Kent - Based on the additional information you've provided, it's possible that the SWR wobble in your 1st plot is due to the impedance bump in the UHF-SMA adapter with additional contribution from your RG-58A/U.

I created another SimSmith model with load data from the ENZEC antenna mentioned earlier (Inv vee, 50+j0 at 340 MHz, 200-500 MHz sweep). I added three sections of transmission line per your descriptions:

14.83 ft of Belden 8219
0.6 inch section to represent the UHF-SMA adapter
6 inches of generic 50-ohm coax.

Belden 8219 uses foam dielectric (velocity factor 0.73) and has a characteristic impedance of 54 ohms. This may or may not be the same as your Berk-Tek cable which you said has a foam dielectric. The difference between 50 and 54 ohms is enough to put a noticeable wobble in the SWR chart.

Why? If the load impedance and transmission line impedance do not match, the transmission line will transform the load impedance. If you view transmission line behavior on a Smith chart, the impedance transformation path rotates in a circle centered on the characteristic impedance of the transmission line. The degree of rotation depends on length. One full rotation occurs every half wavelength. (Loss also makes the path spiral inward, but let's hold that thought.) If the coax impedance is 50 ohms, the impedance transformation will rotate around that value, and the transformed result will stay the same distance from the 50-ohm Smith chart center reference. For example, start with a 25+j0 ohm load and add 50-ohm transmission line in 1/8 wavelength increments. First we get 40+j30, then 100+j0, then 40-j30, then back to 25+j0, then it repeats. All these impedance values have the same SWR because they are all the same distance from 50+j0. Now do the same thing with 54-ohm coax. The rotation will be centered on 54 ohms, but somewhat eccentric relative to the 50-ohm chart center. One side of the rotation will be a little further from the center. The other side will be a little closer. The SWR value will wobble up and down and repeat every half wavelength. In general, the more the transmission line impedance differs from 50 ohms, the greater the eccentricity, and therefore the greater the wobble amplitude.

So maybe your your coax is 50 ohms on the dot? Maybe it's 54? If it's 54, it contributes some SWR wobble.

UHF connectors are known to create an impedance "bump" of about 35 ohms. Inserting a 0.6-inch section of 35-ohm coax in the SimSmith model produces an SWR plot very similar to what you saw. It's remarkable that such a short section could have this much effect, but at these frequencies it makes a difference. It's clearly enough in this case to nudge the eccentricity of the impedance rotation off-center. ?

Plots are attached. There are numerous caveats and nuances to the description above. Hopefully it still gets the main points across.

73, Dave
NU8A
P.S.
The velocity factor doesn't cause the SWR wobble, but it does determine the period .

Dave - Thanks for the additional analysis and plots which are all very interesting. I have noticed the sensitivity of everything involved in these antenna measurements.

I had thought about the SO-239 impedance bump. The 6 inch SMA to SO-239 adapter at the NanoVNA Port 0 connector would introduce that bump when calibrating at Port 0. It should not be a factor when calibrating at the end of the coax. However, the short SO-239 to SMA adapter used for calibration at the end of the coax would add a small discontinuity as you mentioned. Note also that the dipole is built on a SO-239 connector. One element is soldered to the center pin. The other element is soldered to a small ring terminal and attached with a small nut and bolt.

I took Jim's suggestion and measured the phase shift with no connection at the end of the RG-58A/U coax. The frequency was varied to determine where the transmission line is a half wave. The result was approximately 22.65 MHz. I also saw that lower frequency difference between peaks after reducing the width of the sweep.

The 24 MHz I mentioned earlier was the result of having a 300 MHz sweep and only 101 data points. The resolution is 3 MHz per data point.

Your transmission line may be a half wave at a somewhat different frequency which would affect the comparison.

My NanoVNA-H4 does provide 401 data points at lower frequencies but for some reason it only provides 101 data points for the sweep above 200 MHz.

73, Kent
AA6P

There are videos on YT on how to measure characteristic impedances of stripline and coaxial cable.One I watched used a 50 ohm pulse generator on 75 ohm coax.I was wondering whether making the generator 75 ohm instead of 50 makes a difference

There is not much mystery here. Transmission lines that are mismatched at all create impedance transformations at every odd multiple of a quarter wavelength. They become transparent, meaning the impedance at the input is exactly the impedance at the output, at even multiples of a half wave.

Calibration at the input allows you to see the same load your transmitter sees. Calibration at the output will show the actual impedance of the radiator.

Ferrite beads and baluns do not affect either measurement unless they introduce loss or an impedance transformation. Baluns especially are misused as matching devices because, when not properly deployed, they introduce loss. Loss reduces reflected power by a 2X factor and lead to grossly misleading VSWR readings at the input side of the coax.

The nanoVNA is a wonderful tool for both.

Warren Allgyer - WA8TOD

Not just a quarter wave of coax in a mismatch. Any additional length of
coax or transmission line with SWR (mismatch) will produce modification of
impedance. Play with SimSmith using nothing other than additional series
lines in a mismatched condition. It's a good educational tool as well as a
design and evaluation tool.

Installation of ferrite beads and/or choke balun at the feedpoint of a
balanced load will affect the impedance as, without those, the outside of
the coax shield is part of the radiating structure.

Dave - W?LEV

--
*Dave - W?LEV*
*Just Let Darwin Work*

Thanks Charlie, Warren, and Dave W0LEV for the additional help and information. It looks like my original thinking on the two plots was basically correct. The NanoVNA really is a wonderful tool and I've had a lot of fun doing all kinds of experiments.

I now have my NanoVNA-H4 providing 401 data points for the 200 MHz to 500 MHz range. I had calibration information stored for several other antennas and apparently hit a memory limit.

The simulation provided by Dave NU8A is quite impressive considering that my antenna is not in free space and the resonant frequency is somewhat higher than the model. We also don't know the exact characteristics of the antenna, transmission line, and adapters. With 401 data points, the resonant frequency appears to be a little over 344 MHz. The 6 inch SMA to SO-239 adapter uses 50 ohm RG-316 coax.

The antenna is in a large first story room and about 5 feet above the floor on a vertical mast. Very slight changes are detectable if I rotate the antenna so nearby objects must be having some effect. As several people have suggested, the feed line would also be a radiator. Slight changes are seen when touching the coax cable or changing its position relative to the mast.

73, Kent
AA6P

Sorry Dave. Simply not the case. A matched transmission line made of coax does not radiate from the shield. That is why it is called a “transmission line” And not an “antenna”.

A quarter wave and a half wave transmission line, along with a transmission line of any length terminated in its characteristic impedance are all “matched” by definition. Matched transmission lines do not radiate.

Ferrite beads, choke baluns, and other means of common mode rejection are not necessary under matched circumstances. Though not necessarily gimmicks, they are often employed to overcome a poor installation and to mask the real problems.

Warren Allgyer - WA8TOD