Wednesday, September 05, 2012

Tuning up an NA-666

The Nagoya VHF/UHF antennas made in China and available from sellers like 409Shop are popular and cheap, but I think they leave a lot to be desired in the quality control department.

A couple of years ago I bought a Nagoya NA-666 with a regular male SMA connector for use with my Kenwood and Yaesu rigs. I was impressed with the performance of this antenna, and with the fact that it achieved a true 1:1 SWR at bang on 145MHz.

So when I ordered another antenna of the same model but with a BNC connector to work with all my HTs (which have now all been fitted, where needed, with SMA to BNC adapters) I was surprised to find that its performance was a disappointment.

I am well aware of the existence of fake antennas on eBay and have bought more than my fair share of them, but this looked to all intents and purposes to be a genuine Nagoya (silver on black label on the base and a serial numbered Nagoya hologram on the pack.) It had been purchased from 409Shop, a reputable seller. However, when tested on my RigExpert AA-200 antenna analyzer the nice sharp SWR curve dipped to a minimum at 135MHz - 10MHz too low. The SWR at 145MHz was off the scale. Ho hum.

As any ham knows, if an antenna tunes too low in frequency the solution is to cut bits off. After a bit of a struggle the rubber end cap came off and I gingerly pruned about a tenth of an inch . The antenna analyzer showed the minimum SWR point had moved up 1MHz. So I carried on with the cycle of cut, test, cut, test until I had achieved a much more reasonable SWR at 145MHz.
Final SWR curve of the shortened antenna

As I approached 145MHz the antenna was now quite noticeably shorter and I was concerned that I may have passed the point at which the improvement due to a better match was counteracted by the reduced size of the radiating element. I may have passed that point but it is very difficult to make reliable and repeatable field strength measurements. Therefore I didn't make the final cut which would have brought the SWR (shown above) to 1.0:1 at exactly 145MHz.

Field strength measurements and on-air tests led me to the conclusion that the 7-inch shortened NA-666 performed 2-3dB better than my 8.5 inch long NA-701. It beat all the stock rubber ducks by another 2 or 3 dB. The only antennas that outperformed it were a quarter wave telescopic (19 in long) an even longer Nagoya NA-767, a nicely made but unbranded "RH-770" and a "Diamond" RH-205 5/8 wave telescopic, all of which are really too long and cumbersome to use with a small radio like the Baofeng.

Using the shortened NA-666 I have had solid simplex contacts with 5 and 9 reports over distances of several miles and can even access a repeater 50 miles away from inside the shack. So I'm pretty pleased with the result.

1 comment:

Richard G3CWI said...

Very interesting. My NA-666 was resonant at 156MHz and required the addition of 2.5 inches of wire to "resonate" it. Even then the VSWR was 2:1 (which is actually fine in this sort of application). The overall length of the antenna is 11.25 inches. Very strange.