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 ;-)