Saturday, November 22, 2008

EI3EBB, Watson 2000 & antenna work...

After a slightly shorter absence from the airwaves compared to me, Alan EI3EBB is coming back on again. So after he installed a new Watson 2000 vertical on the chimney, it was time to help out with a bit of tuning.

The EI3EBB 'antenna farm'. Diamond X510 on the left and a Watson 2000 on the right.

The Watson 2000 is a 3 band fibreglass VHF antenna. On 50 MHz, it is a half wave (claimed 0dBd gain), on 144 MHz it is a 2 x 5/8 wave (4dBd claimed) and 432 MHz, it is 4 x 5/8 wave (6.4 dBd). The overall lenght is 2.5 metres so the claim that it has the same gain as a half wave (3 metres) on 50 MHz is a bit optimistic.

Adjusting the 50 MHz radial on the Watson 2000

50 MHz......It has 3 ground planes with one of them adjustable for tuning the antenna to 50 MHz. The initial test showed that the VSWR was lowest up around 51 MHz. After adjusting the radial for maximum lenght, the lowest VSWR was at 50.7 MHz. The VSWR plot was as follows...

The 1:1.5 VSWR bandwidth seems to be around 0.9 MHz and the 1:2 VSWR bandwidth was 1.5 MHz. The final result is that the VSWR for the SSB portion of the band is 1:1.4 to 1.6 which should be ok. It's not that we had much choice when it came to tuning anyway because if we had adjusted it any further, the radial would have fallen out!

144 MHz....The VSWR meter suggested the following...


This suggests that the aerial was slightly too long. Certainly, the VSWR looks a bit high for the FM part of the band where this type would normally be used. As there was no way of adjusting the VSWR on 2 metres, we had to leave it as is. Did adjusting the 6 metre radial affect the VSWR on 2 metres?? We didn't check.

432 MHz....We had no equipment to check the VSWR on this band.

(Note...The VSWR was checked with a pretty cheap meter. A days later, we took some readings with another meter. While there was some differences, the VSWR minimum point stayed the same. There was nothing to suggest that the graph above is not accurate)

50 MHz.....It's way too early to say how this aerial performs on 6 metres. The only way we had of testing it was to listen to the EI0SIX beacon and compare it to how I hear it at my place. On paper, we both expected the beacon to be a lot stronger at EI3EBB's location. He is roughly double the height above sea level that I am and he is near the top of the range of hills that I have as my horizon. In addition, he is using an aerial that is resonant and should have nearly the gain of a dipole while I am just listening on a non-resonant CB half wave. If I was hearing the beacon at 319 to 419, I would have expected it to be maybe 539 at EI3EBB's place?

Instead, the beacon is very weak at his house. Why??
Is it the Watson 2000? It's hard to believe it's that bad. Anyway, it seemed to have got a good review here.
Different path???.....I looked at some maps and we have almost the same path (EI3EBB - 168kms - 38 deg / EI7GL - 183 kms - 34 deg). If anything, the path for Alan looks slightly better in that the Knockmealdown mountains look like more of an obstruction for me.
Local obstructions???......EI3EBB has some high ground to the East but at 38 degrees, he should be clear of most of it. Is there some sort of 'plateau' effect?? Is the aerial just in a null spot?

Updated 25th Nov 2008......We did a few tests by going mobile around the roads near EI3EBB's location. We used a loaded quarter wave magmount on the roof of the car and an IC706 parked on 50.052 MHz CW. Now, the tests were a bit subjective as we had ignition noise, noise from the car's electronics and there was some fading on the signal as well. Yet, we still got a good feel for what could be heard.

Area 1) Around EI3EBB's location which is about 500 metres back from the brow of the hill and at ground level, would be around 5 metres lower. The signal was very weak, maybe 319, 419 max.

Area 2) Near the peaks of the local hills (~2-3kms away, 40m higher and 15m higher) to the East and West of EI3EBB. Here the beacon was around 539, nice solid signal and a lot stronger than with the Watson 2000 at EI3EBB's place. (On the hill to the East, the signal did not really get strong until we cleared the trees)

Area 3) The strongest signals by far were along the Northern slopes of the Eastern hill. The signal peaked around 559 here, amazingly strong for a beacon that was around 165 kms away. I have seen this with VHF signals before where the signals on the downward slope of a hill in the direction of which the signals are coming are often stronger than at the top.

Conclusions....??? Still nothing definite. We established that the Knockmealdown mountains (direction of EI0SIX) are visible from the Watson 2000 on the roof. If there is any 'local' high ground in the way, it's not much. We tried just holding the magmount out a skylight on the roof so that it was close to the height of the Watson and doing some tests by comparing it to the Watson with a coax switch. We could hear the beacon a little bit better with the Watson. So, in other words, we were hearing the beacon 'a bit' better on what is supposed to be a resonant half wave antenna than a loaded quarter wave with no ground plane. Perhaps it's a bit early to come to any firm conclusion on the Watson 2000 but for the moment, on 6 metres at least, there is certainly no 'wow' factor.

144 MHz......Compared to the Diamond X510 which is nearby, the Watson is a bit down as expected. The only tests done were listening to local repeaters and they seemed fine.

(Notes....Limerick Repeater on IC706. X510=s'9+10', Watson=s'9').

432 MHz.....No tests done.

So, EI3EBB in IO52ta is back on 6 metres...

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