Testing short HT ant with NANO VNA?

[dkcorlfla]

Hi all, something that puzzles me is how does a short HT antenna work and how to test it using a NANO VNA. What I do not understand is if it's a dipole or 1/4 wave ground plane where is the other half of the antenna? Is it using the chassis of the HT for the other half? If so how would someone replicate that using the NANO VNA?

I'm also interested in testing small antennas used on micro controller LoRa boards. I calibrated for 860Mhz > 960Mhz two times, once to the SMA on the NANO and the second time to the end of a 6 inch long coax. The results where all over the place, no resonance found and the SWR at the top with only a slight dip.

Seems to me the antennas must be a 1/4 wave with a missing half as anything else like an end fed would not have anything close to 50ohm impedance at the feed point.

Any ideas?

 

[k6cpo]

Trying to test the SWR of an HT antenna with a NanoVNA or an analyzer is an exercise in frustration. The unmounted antenna is going to behave differently than it will when connected to the radio. I even tried creating a "fake radio" using an aluminum project box in an attempt to replicate the antenna being attached to a radio and that didn't work either.

 

[merlin]

K6CPO makes the best explanation of errors using a Nano VNA to look at antennas.
My best results were with SMA mounted antennas and using an adapter straight into the channel 0 port.
This means not having anything conductive near the AUT. thats like 18 inches for VHF. even hand/body position will affect results.
If the antennas don't use SMA (like Kenwood) you need to get an adapter to go to SMA. shortest distance between antenna and Nano.
Same setup with the LoRa provided the antenna is separate with SMA connectors. this gets critical above 500 MHz.
You can hold the Nano VNA in hand with the antenna well in the clear, That will act as your counterpoise.
Results are not perfect, but close enough for average user outside of a lab.

 

[merlin]

Trying to test the SWR of an HT antenna with a NanoVNA or an analyzer is an exercise in frustration. The unmounted antenna is going to behave differently than it will when connected to the radio. I even tried creating a "fake radio" using an aluminum project box in an attempt to replicate the antenna being attached to a radio and that didn't work either.

That should work acceptably and with your run of coax from your jig to the VNA, you will need to calibrate that extra coax.
The you will have to hold the jig while checking the antenna. Note, in the clear as I mentioned above. (3 hands would be nice.)

 

[merlin]

I calibrated for 860Mhz > 960Mhz

You might try a broader sweep span, say 700 to 1000 MHz. that will give you a better plot on the S11 and Smith chart.
Don't forget to calibrate first.

 

[mmckenna]

Here's how I've tested some. It's an old Kenwood TK-290 with the innards removed and a test cable soldered to the underside of the SMA connector on the top of the radio. It falls in the "good enough for testing" category. It's not "lab grade" and isn't intended to be. It's for testing unmarked antennas, suspect antennas, etc. It works well enough for my intended purpose.

The radio chassis acts as the counterpoise for the antenna, and there is some amount of capacitive coupling to the user holding the radio. I can make the resonance of the antenna change by moving my grip on the radio.

The hand held antennas come in all kinds of variants. Some are 1/4 wave, but I've had Motorola 1/2 wave dipoles, stubby/loaded antenna, and others.

 

[prcguy]

Here's how I've tested some. It's an old Kenwood TK-290 with the innards removed and a test cable soldered to the underside of the SMA connector on the top of the radio. It falls in the "good enough for testing" category. It's not "lab grade" and isn't intended to be. It's for testing unmarked antennas, suspect antennas, etc. It works well enough for my intended purpose.

The radio chassis acts as the counterpoise for the antenna, and there is some amount of capacitive coupling to the user holding the radio. I can make the resonance of the antenna change by moving my grip on the radio.

The hand held antennas come in all kinds of variants. Some are 1/4 wave, but I've had Motorola 1/2 wave dipoles, stubby/loaded antenna, and others.

You would want to add some ferrites to the coax as it exits the housing to decouple the coax from the housing so the long coax braid/counterpoise doesn’t skew the measurement. In my opinion connecting an HT antenna to a nano VNA then holding it in your hand will simulate an HT environment close enough.

 

[RFI-EMI-GUY]

Motorola SABER radios and others (MX Mount) used a UHF helical antenna that was ubiquitous and sold under a specific part number.

Unfortunately with age, very prone to cracking and looking awful.

Some years back i put in an order to /\/\ national parts for a bunch of them. What I got back was a new antenna under a new part number.

Right off the bat I noticed that they did not receive as well at 462 MHz. So I contacted /\/\ and eventually talked to the vendor himself. He claimed it was "same antenna, made under same 12M specs, yada yada tuned at 414 MHz" WTF???? It is supposed to be a 450-470 MHz antenna. Oddly, it is even a bit shorter than the original. They sent me some replacements, same issue. I noticed the new ones listed as 440-470 MHz on a dealer website, so maybe the new model was to please the hammies.

There is indeed a special sauce to making these things and an even a very special way to test them.

A thought, you could take two identical antennas and use the second one as a counterpoise and test with them supported on an insulated pole in free space. That might add some natural order to the test. But the reality is that once mounted on a radio, and in your hand, it will tune differently.

 

[dkcorlfla]

Thanks for the interesting replies. I wonder if it might be possible to test the counterpoise effect of the hand held radio using the NANO VNA and then be able to more closely replicate it for better antenna testing? I might have to play around with that.

Regarding the very small LoRa boards that I'm trying to test the little antenna on the thought came to mind that the USB-C cable being used to power the thing might be acting as a counterpoise so I tested it with the DMM and sure enough both cables I was using on both of the LoRa boards had continuity from the antenna hex nut (shield) to the USB computer plug. Interesting that a third cable I tested did not.

Not sure what happens when the board is powered by a battery but I plan to test the neg side of the battery wire.

 

[dkcorlfla]

Not sure what happens when the board is powered by a battery but I plan to test the neg side of the battery wire.

I just tested one of the boards and the negative side of the battery connector/wire that came with the unit does have continuity to the hex nut on the base of the antenna so it looks like it and the battery would provide some counterpoise.

On a side note I have heard of people hooking up a battery and burning up the board because the battery pack had the wires reversed. Checking the pins on the onboard connector and finding the one that has continuity to the antenna hex nut might help prevent this. The board (Lily Go T3S3) also had a + and - that were correctly labeled.

 

[vagrant]

In my opinion connecting an HT antenna to a nano VNA then holding it in your hand will simulate an HT environment close enough.

Agreed. I have tested over two dozen handheld antennas using a handheld analyzer. When the analyzer was sitting on top of a table the numbers were ugly. When I held it in my hand the numbers became significantly favorable to the bands the antennas were designed for. Well, some antennas were junk. Anyways, I must note that the handheld analyzer I used has a metal case.

It is similar to this particular analyzer in design, but not coverages nor features.

Amazon.com

 

[dkcorlfla]

Agreed. I have tested over two dozen handheld antennas using a handheld analyzer. When the analyzer was sitting on top of a table the numbers were ugly. When I held it in my hand the numbers became significantly favorable to the bands the antennas were designed for. Well, some antennas were junk. Anyways, I must note that the handheld analyzer I used has a metal case.

It is similar to this particular analyzer in design, but not coverages nor features.

Amazon.com

Thanks for the reply, you just gave me the idea of using a HT size piece of aluminum flashing attached to the SMA connector on the NANO VNA to approximate the antenna being mounted on a HT instead of the NANO. I have some OEM antennas that came on on the Baofeng UV-5G GMRS radios and I'm curious what they look like on 440Mhz vs 462 ham/GMRS