Mobile Anrtenna off the charts. And possible fix?

[GreekMan]

The antenna is a Nagoya NL-77B mobile V/UHF antenna
http://www.409shop.com/409shop_product.php?id=102336
http://m1avv.blogspot.gr/2013/05/mobile-antennas.html

The story is this....
Some years ago, me being a green HAM, I had mobile connector problems on a trip and ended up in a HAM friendly technician's shop.
He proceeded to test antenna and cabling with a meter (SWR?) and said the antenna was actually an Airband antenna!!!!

Fast forward last week with me testing various antennas with my own SWR meter.
And while readings were reasonable with all other antennas, the NL-77B one was of the charts

MHz - SWR
137 – 2.2
145 – 3.4 (forward 4.8 & return 1.42 watts)
168 – 3.9
Seems like it gets better the lower it gets, but here are the UHF measurements
440 – 2.3
450 – 1.7
456 – 1.62
460 – 1.38

Question is what to do with the antenna?

The main element seperates from the base with 2 screws and I could shorten it, but I do not know how the wavetrap in the middle would affect with the tuned frequency.

BTW HAM bands in Greece are: 144-146 & 430-440 MHz

[Tevin]

It looks like what you really have is a UHF antenna marketed as a dual-bander, and honestly, the SWR readings you get for your part of the UHF band are just as bad.

Shortening it would actually make the performance worse.

As for what to do with it...I'm not sure what antennas cost in Greece, but here in the USA they are pretty cheap. I would keep that antenna for my scanner or some other application and go get a new antenna that you know will work on the bands you are using.

[Tempstar]

What Tevin said.....

[Rick]

What SWR meter are you using?

Consider it training.

You have a dual loaded antenna, The middle load is the 440. There is an other loading coil in the base, for the 2 meter band. The issue with loading coils is that they seldom cover the whole band. The nice thing about loading coils is that you can get "gain".

To adjust:
Start with the top portion of the antenna, there are two set screws in the middle loading coil, adjust the top portion to the lowest SWR in the 440 band. You will find the antenna is it is not seated all the way down, and you have a 1/2 inch adjustment up or down. Use marks to keep track of were you are on the antenna.

Now adjust the 2 meter portion. For this adjustment use the set screws on the base. Adjust the whole antenna for the lowest SWR. Again you will have about 1/2 inch of adjustment. Again make marks.  If you need to make the antenna longer, there is a second set of screws on the bottom of the middle loading coil that can be used to make the 2 meter antenna longer.

Go back and check the 440 SWR, it should have stayed the same.

[GreekMan]

that is very helpful....

Unfortunately there are no set screws on the middle loading coil (we use the term wavetrap here).
So I will have to adjust for VHF and see how it goes.

I will be saving your instructions for future use, my Diamond 770 moble antenna is full of adjustment screws, so the process may be be used in the future...

Thanks!

[Rick]

I would have sworn the showed in the picture.

The Diamond and my Comet both have full set of screws.

[GreekMan]

no...it is visible in the 409shop pic that there are only on the base...
Anyway tommorow will be testing day no3

[Cindy]

I would venture to say that the manufacturer made some substitutions along the way (coil for example) and the resulting product is out of design specs.  Not uncommon with Chinese sweat shops.

I'd say you have a nice start to a "project parts pile."

-Cindy

[Rick]

Yes its a nice project pile.

It will be interesting to see if he can get the 2 meter portion working.

I use a MFJ-269 for my antenna adjusting, have not done it old school in a while.

[GreekMan]

I'd say you have a nice start to a "project parts pile."

hmmmm...like throwing away the rest, keeping the bottom loading coil and making several lengths of antenna to work various VHF frequencies?

[spacecase0]

you could make an extension between the base and the antenna whip you already have 

[GreekMan]

Update:

I used the adjustment range to lengthen the antenna abit. i got better readings in UHF, but by not much.
So Tevin is right.....

So now the road is open to cut it shorter and see if I can make it of use in the VHF band.

Also assuming I try a new whip out of bronze soldering rod, what would your guessimate of starting length would be?

[GreekMan]

I am perplexed...

To adjust:
Start with the top portion of the antenna, there are two set screws in the middle loading coil, adjust the top portion to the lowest SWR in the 440 band. You will find the antenna is it is not seated all the way down, and you have a 1/2 inch adjustment up or down. Use marks to keep track of were you are on the antenna.

If I am getting it right, Rick says that it is the portion of the antenna Above the loading coil that works on the UHF

But in this video the guy says it is the lower part

https://www.youtube.com/watch?v=_qw77VIs44s

[spacecase0]

I am pretty sure that the higher frequency section of the antenna is between the antenna connection and the loading coil.
the point of the inductor is to block the higher frequencies

[GreekMan]

then extending the lower portion will improve both bands....
TNX