Dielectric loading of rain drops...?

[VK4MIL]

Well that could be problem , but if a device cannot handle 1:8 or there about send it back

[VK2XV]

Added real rain SWR curve to...

http://www.vklogger.com/forum/viewtopic ... 592#p51592

[VK2XV]

Well that could be problem , but if a device cannot handle 1:8 or there about send it back

Are you offering to pay the freight back to the US ?

Seriously - the linear hasn't arrived yet - promised to be a 2 week delivery time, now 5 weeks... I will read the documentation when and if it arrives. I am sure it can be managed in some way by being careful. (Hmmm - we need a fingers crossed smiley).

[ZL1RS]

Your SWR plots are telling ... that is a relatively "narrow SWR bandwidth" Yagi (+ or - any effect that the gamma match system might have on the measured SWR). The manufacturer's web site claims 11.8dBd gain on a 4.05m boom which is a fairly high gain for the boom length and would probably be achieved at the sacrifice of SWR bandwidth (and it possibly has quite a low 'natural feed impedance', hence the use of a gamma match).

Compare the SWR chart with another design having the same 11.8dBd forward gain, but a wider SWR bandwidth design. The boom length on this example is 4.4m

144MHz 9 element Yagi

Note: the same +-1600kHz SWR bandwidth plot as yours (but centered on 144.300MHz rather than 144.280MHz).

To simulate the effect of rain drops "lengthening" the elements, here is the SWR curve for the same antenna with elements 4mm longer

same Yagi with 4mm longer elements

The 4mm was only chosen as it happened to move the SWR dip down the band by the same 400kHz that your antenna's SWR dip moves ... the actual effect of rain water on this example Yagi may be different. But the point to note is the that SWR is still very low (1.09) due to the wider SWR bandwidth design. And this example antenna does not have a particularly wide SWR bandwidth, it is possible to optimise a Yagi design for much wider SWR bandwidth (e.g. <1.1:1 over 2 MHz or more at 2m is achievable). Of course there are trade offs, and gain is usually the one focused on ... but would a couple of tenths of a dB be noticed in real life?

As a matter of interest, in the simulation the gain of this example antenna increased a few hundredths of a dB with the longer elements (this is frequently the effect of slightly longer elements).

FWIW, the 9 element 2m Yagis in my portable EME system suffer a similar rain water SWR shift as yours. The HB LDMOS amplifier keeps chugging along (the device is rated at 1200W but I run it at only 800W in the forlorn hope that the lower power is somehow a 'safety margin') ... however, the SWR change does prey on the operator's peace of mind! As soon as the rain stops I go out and give the mast a shake so the water droplets fall off the elements and the SWR goes back near enough to 'normal' ... I feel better, but the amplifier probably 'doesn't care'

73,

[VK2XV]

G'day Bob,

Your SWR plots are telling ... that is a relatively "narrow SWR bandwidth" Yagi (+ or - any effect that the gamma match system might have on the measured SWR). The manufacturer's web site claims 11.8dBd gain on a 4.05m boom which is a fairly high gain for the boom length and would probably be achieved at the sacrifice of SWR bandwidth (and it possibly has quite a low 'natural feed impedance', hence the use of a gamma match).

I guessed as much - getting the gamma match tuned was a bit sensitive. As you say the high gain/boom length is a clue.

FWIW, the 9 element 2m Yagis in my portable EME system suffer a similar rain water SWR shift as yours. The HB LDMOS amplifier keeps chugging along (the device is rated at 1200W but I run it at only 800W in the forlorn hope that the lower power is somehow a 'safety margin') ... however, the SWR change does prey on the operator's peace of mind!

Yes - I am worried even before the LA has arrived - maybe it is the dollar signs...

As soon as the rain stops I go out and give the mast a shake so the water droplets fall off the elements and the SWR goes back near enough to 'normal' ...

Another version of the Armstrong method...

Thanks for the info.

[ZS6GST]

Wonder if you have considered applying some high quality canuba wax polish on the elements?
That should make the drops drop off. There is also some natty liquid treatment meant for windscreens that may work,

73 de Pine, ZS6GST

[VK5ZLR]

In the apparently unlikely event that it ever rains here......I shall study this thread in detail !