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Wednesday, September 12, 2012

Using the V2000 triband antenna on 70 MHz band

During the first year the 70 MHz band was authorized in Spain some spanish hams were active in the band using PMR transceivers in FM. Curiosity made that I connected my IC-91 walkie with general coverage receiver to the V2000 antenna just to see if something can be heard. To my surprise, I was able to hear a lot of G stations in the first days, with some locals also. Signals were quite strong, too strong for a no 70MHz antenna so I stated to wonder if the antenna could be usable on 70 MHz also.

After some time I acquired a small 70MHz FM PMR walkie so I was able to measure SWR: A bit over 5.

A SWR value of 5 is quite high, but the strong signals received encouraged me to try to tune the antenna using a quick and dirty tuner made in prototype board:




After some trying with the number of turns in the coil I reached the match, at almost 1:1. Results on air were quite good. I was able to make some local contacts with 5W using FM. Signal report were good so there was no doubt the antenna can be used in 70 MHz with the aid of a tuner.

The next step was to made the tuner more robust and usable. I used a small aluminum box from a TV antenna amplifier with two variable capacitors from pocket radios using pi configuration:



The other side of the box:


I use the tuner at the 50 MHz port of a MX-2000 triplexer. 70 MHz is just at the edge of the HF/50 MHz filter cutoff so it can be used safely. Maybe you have noticed I didn't attach knobs to the capacitors. There is no need to add them. Once you move the capacitors to get SWR of 1:1 on 70.200 MHz, there is no need to readjust them again.

Tuning at 70.2 MHz bandwidth is about 300 kHz (from 70.1 to 70.3 MHz) with SWR under 1.5 so because in Spain actually the authorized range is 70.150 to 70.200 I can use the V2000 antenna in the 70 MHz band at any authorized frequency without the need to retune.

This is my current station arrangement:

So when I want to work on 70 MHz I only need to switch the triplexer's output to the transverter and select the 28 MHz band in the FT-817. That's all.

I have tested my particular setup with powers up to 25W in 70 MHz without any problem or antenna degradation.

I would not recommend you to use high power on 70 MHz band with the V2000 antenna.

I used up to 20-25W with no problems but there is possibilities to burn one of the V2000 capacitors in the matching section (at the base), so be warned.

Does it work? Yes. maybe (sure) its performance is lower than a dedicated 70 MHz antenna but it works fine. Most countries with 69/70 MHz allocation are within one sporadic-E hop from my QTH so they are easy to work: EA8, CT3, I, DL, PA, 9A, S5, OK, ON, SP, G, EI, etc, etc... It works also nicely with tropo signals, having ranges similar to those at 6m band.

So if you have a V2000 triband vertical antenna and you are interested in the four meter band, give it a try. It's better than nothing ;-)




5 comments:

  1. Miguel
    Very interesting!

    I would like to try this but I have two questions for you if you do not mind:-
    1) what was the configuration of your ATU - Z match or something?
    2) was this using your revised V2000 with the 50MHz ground plane?
    I ask because I may instead fix a 70MHz ground plane but I am not sure how this would work and if the 50MHz GP works I might do that instead. I would not use it for 50MHz, just 70/144/432. I have a Comet diplexer which should work.
    I expect the efficiency to be low but then under Sporadic E you do not need much efficiency. The local stations may have to accept what they get. For me, I would have one less antenna!
    Thank you very much for an interesting site.
    Jim
    GM4FVM

    ReplyDelete
    Replies
    1. 1- The tuner was in typical pi configuration: C - L - C

      2- Yes. My V2000 antenna always use the 50 MHz radials. If you are not going to use the 6m band, you can try to cut the radials to 70 MHz. This should help.

      Good luck!

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  2. Yes of course I see what you did with the ATU. Thanks.
    I wrote to Diamond and they now say they are looking at a multiband antenna for 70MHz. If they ever do this - I will certainly do a 70MHz groundplane like yours. Thanks for that.
    For now, I should like to try that ATU for the V2000.
    Best wishes. Thanks for a great site.
    Jim GM4FVM

    ReplyDelete
  3. Hi Miguel...
    I am about to give this a try. I was wondering what approximate values you have for the capacitors & inductor? I guess I could estimate the inductor from your photos, but if I knew the capacitor values it would help.
    Thanks for the project write-up.
    73 de Gerry

    ReplyDelete
    Replies
    1. They are pocket radio capacitors, and I'm using the FM section. Don't take my tuner as a example, just as a starting point: Your coaxial length is different that mine, so the impedance you see can be dramatically diferent from the one I see.

      Delete