Dielectric loading of rain drops...?

[VK2XV]

Noticed the SWR on my 9-el 2M beam was up to 1.6:1 this morning on 144.230 MHz while running FSK441 M/S. Thinking the worst I measured with an antenna analyser and found the SWR was fine - except the minimum point had moved down from 144.220 MHz to 143.850 MHz - almost 400 kHz... I wandered outside to see if some damage had occurred - none seen from the ground. However, there were raindrops hanging from all the elements from the overnight rain (you remember what rain is don't you ?). "Surely this is not the cause" I said to myself - "when the Sun burns them off I will see". Sure enough with the raindrops evaporated the SWR is back to 1.05:1 at 144.220 MHz as before.

I expected some shift and/or change with water on the beam - but I am surprised about the magnitude.

Comments ?

[VK4ADC]

Steve

As well as raindrops, the plastic end caps cause a significant frequency shift. If you add them to an existing yagi that hasn't been designed with them in mind, you will find the frequency shifts down several hundred KHz on 2M, sometimes MHz on 70CM.

The converse applies too: an antenna that was originally supplied with end caps shifts up if the caps weather and fall off after a few years exposure to the sun's UV. You need to put replacements on to maintain the original characteristics - hopefully using the same composition and style caps as the originals to minimise the frequency shift.

As for rain, we could do with some of that on a more regular basis to keep our tanks filled... regardless of the effects on antenna tuning..

[VK7JG]

Hi Steve.
I had exactly the same problem with a 6M7JHV .

When it started to rain the SWR went up some what . However if it kept raining it would come back down a little .
Like you I thought there was possibly a problem with the coax .
I am using the same coax run with a 6M8GJ without any problems .

73
Joe
VK7JG

[VK6ZFG]

The impact will be more on a yagi using thin elements than one with thick elements as the thin elements have less bandwidth than the thick ones so will go out of tune more readily.

[VK2XV]

Thanks for the comments.

It rained heavily overnight and so I expected the SWR to be high this morning - but to my surprise it was as normal.

Wandering outside for an inspection revealed no raindrops drops visible on the elements. Not sure why - maybe there was sufficient wind with the rain to clear them - certainly wasn't the Sun. The driven element has a gamma tee match so maybe it is a function of what persists on the insulation between the gamma tuning rod and the surrounding tube ?

I checked to see if the SWR is affected by the rain-soaked trees which surround the antenna. The SWR varied between 1.06:1 and 1.1:1 as the beam was turned the full 360 - so that is not the source.

In any case I see that I will have to be vigilant as Santa is bringing me a TE-Systems Linear which apparently has no SWR protection...

[VK2XV]

As well as raindrops, the plastic end caps cause a significant frequency shift. If you add them to an existing yagi that hasn't been designed with them in mind, you will find the frequency shifts down several hundred KHz on 2M, sometimes MHz on 70CM.

Doug - yes I have seen that on the 5-el I was playing with before.

The converse applies too: an antenna that was originally supplied with end caps shifts up if the caps weather and fall off after a few years exposure to the sun's UV. You need to put replacements on to maintain the original characteristics - hopefully using the same composition and style caps as the originals to minimise the frequency shift.

Mmmm - maybe I should get some spares from the antenna supplier (Duoro)... I didn't think of that - thanks for the heads-up.

[VK3DXE]

Thanks for the comments.

It rained heavily overnight and so I expected the SWR to be high this morning - but to my surprise it was as normal.

Wandering outside for an inspection revealed no raindrops drops visible on the elements. Not sure why - maybe there was sufficient wind with the rain to clear them - certainly wasn't the Sun. The driven element has a gamma tee match so maybe it is a function of what persists on the insulation between the gamma tuning rod and the surrounding tube ?

I checked to see if the SWR is affected by the rain-soaked trees which surround the antenna. The SWR varied between 1.06:1 and 1.1:1 as the beam was turned the full 360 - so that is not the source.

In any case I see that I will have to be vigilant as Santa is bringing me a TE-Systems Linear which apparently has no SWR protection...

You'll most likely find that after the first rain event, the crap built up on your elements increased the observed effect, after further rain, it gets washed off and things return to relative normality.

[VK7HH]

Steve

As well as raindrops, the plastic end caps cause a significant frequency shift. If you add them to an existing yagi that hasn't been designed with them in mind, you will find the frequency shifts down several hundred KHz on 2M, sometimes MHz on 70CM.

Does the same effect happen using RTV silicon sealant in the ends of the boom and elements? Doing this tends to make the antenna whistle less in the wind.

Further on the effects of the rain, do you have the feedpoint in a box, or just open air with silicon around it? A lot of problems can also come from boxes at the feedpoint, condensation buildups inside and eventually can get into your feedline. Perhaps when it dries out the effect is not noticeable anymore. Drain holes help.


Hayden

[VK2XV]

You'll most likely find that after the first rain event, the crap built up on your elements increased the observed effect, after further rain, it gets washed off and things return to relative normality.

This could be a factor - I have a vague recollection that I noticed white drip marks when I first put an antenna up - trouble is I cannot remember whether it was this 9-el or the previous 5-el. The 9-el was shiny-new having being made up to order locally (Sydney). Perhaps as time goes by it will be covered in an oxide layer with may reduce the effect - but then again, if it is dielectric loading then the oxide layer might not be a mitigation. Time will tell.

I will attempt an SWR measurement while it is p-p-p-precipitating down - although the last few opportunities were whilst MN's sparklers were on...

[VK2XV]

Does the same effect happen using RTV silicon sealant in the ends of the boom and elements?

The maker (Duoro) actually commented that he uses this method for his personal units. But his were cut for no caps. He found plugging with sealant didn't alter the tune (I presume because it is is not in the path of the electric lines of force emanating from the elements). However, in my case, the antenna was supplied with end-caps so I assume it was cut taking them into account.

Might do a measurement when I have the motivation for taking down and putting up a perfectly working antenna several times. At 65 these times are rare...

[VK4ADC]

Plugging inside the end of an element won't change the tuning but protruding quantities might. Just remember that plastic end caps are insulators too and they affect the tuning so RTV silicone sealant may affect in exactly the same way. Make sure you don't use too much RTV (weight-wise) and cut it off flush with the end of the element. A small plug is all that is needed to quieten a 'noisy' element - from wind passing the open end, usually perpendicularly - and making the whole element vibrate.

What I suspect is that the use of plastic caps alters the "end effect" from the typical 0.97 to 0.98 down a little, maybe to 0.96 to 0.97. The effect is certainly repeatable so what I have tended to do when building multiple yagis of the same type is to build one & measure it, add the caps on all elements and remeasure. Take off all of the caps and trim all of the elements by a small measured amount (eg 2mm), add the caps again and remeasure. That gives you an appreciation of the frequency shift per millimeter (or two) change in element length for that band and for those styles of materials & construction when caps are used. You can then calculate and cut to the desired centre frequency with a reasonable level of confidence, applying the caps afterwards.

If you are going to build multiples antennas of a single design, you will still see a variation in centre frequency due to very small differences in element lengths or offsets from centre (even 1mm makes a difference) during assembly or even just the testing jig putting the antenna slightly different angle or height above ground etc.. I always test my yagis mounted directly vertical without overhanging tree branches and with the reflector at least 1.2m above ground for 2M and 70CM designs, non-metallic support frame too. Just those overhanging branches can (and will) affect your results.

[VK2XV]

and cut it off flush with the end of the element.

Yes - this was the method described to me by Peter (Duoro Antennas).

[VK3ZAZ]

Noticed the SWR on my 9-el 2M beam was up to 1.6:1 this morning on 144.230 MHz while running FSK441 M/S. Thinking the worst I measured with an antenna analyser and found the SWR was fine - except the minimum point had moved down from 144.220 MHz to 143.850 MHz - almost 400 kHz... I wandered outside to see if some damage had occurred - none seen from the ground. However, there were raindrops hanging from all the elements from the overnight rain (you remember what rain is don't you ?). "Surely this is not the cause" I said to myself - "when the Sun burns them off I will see". Sure enough with the raindrops evaporated the SWR is back to 1.05:1 at 144.220 MHz as before.

I expected some shift and/or change with water on the beam - but I am surprised about the magnitude.

Comments ?

Its not just ham ant I have seen an FM stack swr drop from 1.4 down to 1.1 in rain with 50KW running through it

[VK2XV]

Did I say I was noticing the effects of raindrops - how about horizontal rain and flying branches and other debris ?

Biggest storm we have seen here in the 40-odd years we have lived here. Fortunately no trees down out of the many on the block, but the roof of my 6m x 3m workshop shed is in the garden and the walls buckled. No radio for a few days...

On a brighter note - all antennas appear to have survived. The 2M 9-el beam is showing the normal SWR curve - but I will have to wait for full light to see if any elements have been bent by flying branches. Didn't have time yesterday - too busy moving and drying electricals from the de-roofed shed.

Our caravan skidded about 1/2 metre along the drive despite locked brakes...

First time I have heard the wind with a low-frequency rumble - like a convoy of heavy trucks...

Scary stuff for a southerner...

Only 80 kmh recorded at weather station 5 km away, but 23 mm rain in 10 minutes. Hate to be here for a really big one...

Power only off for about 6 hours - we are impressed...

[VK4TI]

The subsonics in those large storms makes the hair on your neck stand on end , like a banshee screaming , I remember this one https://www.niwa.co.nz/our-science/coas ... news/winds the Wahine going down like it was yesterday , pulling people out of the sea in the freezing cold and watching people tumbling across the beach was intimidating . I cant say I have ever experienced anything worse in a storm as the bursts over 200 klm/h were indescribable .
I hope and pray this coming summer is more moderate than the forecasts suggests .

[VK2XV]

Did some tests this morning to try and isolate the source of the effect rain has on the SWR of my 9-el 2M beam. Now that it has stopped raining I used a Karcher hi-press unit in thin stream mode to selectively rain on the antenna.

The results were not what I hoped for (isn't that always the way ? - just when you have formed your own pet theory or ideological line in the sand, science comes along to mess it up...).

Here is the dry antenna's SWR curve...

2M_beam_dry.gif (34.91 KiB) Viewed 9368 times

Drenching the driven element and the gamma match did not change the SWR curve significantly.

It was only when I drenched the whole antenna that I could see an equivalent frequency shift downwards in the SWR minimum point as produced by real rain...

2M_beam_wet.gif (33.45 KiB) Viewed 9368 times

EDIT: Just started to rain again and so here is the real rain SWR curve...

2M_beam_rain.gif (32.34 KiB) Viewed 9328 times

Disappointing as I was hoping to just make a rain hat for the gamma match section of the driven element - make a rain hat for the whole antenna would increase the wind load...

2M_beam_umbrella.jpg (8.55 KiB) Viewed 9368 times

It seems the sensitivity is a function of the antenna as a whole and so changing the driven element feed to some other topology would serve no purpose.

I will have to live with it...

[VK5PJ]

Hi Steve,
not sure if you were able to shoot the water above the yagi and let it fall downward onto the yagi or whether you could only wet the bottom side of it? Maybe just maybe there is a sensitive spot on the top side that is impacted from rain falling down and not sensitive to water from below?

Hope you find something as the changes in SWR you mentioned earlier in the thread would be hard to live with especially if there was an opening to a dx location and rain at the same time

Or at another angle try inverting the yagi to see if that improves things for you, of course that depends on how accessible it is to do that.

regards,
Peter, vk5pj

[VK4MIL]

Just a standard calls 2 cents worth but really would it make that much difference to the performance of the antenna?

[VK2XV]

G'day Peter

not sure if you were able to shoot the water above the yagi and let it fall downward onto the yagi or whether you could only wet the bottom side of it? Maybe just maybe there is a sensitive spot on the top side that is impacted from rain falling down and not sensitive to water from below?

The Karcher was used to shoot the elements from directly underneath and so the bottom was wet, but also the top as water going past the elements fell back onto the top of the antenna. As best as I can determine from this experiment it is a whole antenna effect. It is an element-through-the-beam (non-insulated) type and so there is no asymmetry wrt. to top and bottom for the elements (except for the driven element with the gamma T-match). The driven element + gamma T-match was singularly unaffected by the drenching. I tried a number of times to get the drenching of the DE+GTM to cause a shift of the SWR curve - but to no avail (trying to bend science to my wishful thinking). It was only the full antenna drenching which produced a shift.

One of these days I might build my own design using the LFA topology to see how it compares.

Thanks.

[VK2XV]

G'day Colin,

but really would it make that much difference to the performance of the antenna?

You are right - not so much the antenna performance, but modern transceivers start to wind back the power when the SWR goes above 1.5:1 - so no great inherent loss at the antenna itself, but the increased line loss (tiny) + plus reduced power out might be significant.

What really concerns me is that when my gold-plated ($1500 worth) linear amplifier arrives, which has no SWR protection, I don't want to fry it if a sudden downpour occurs. I will need to be extra careful.