[TowerTalk] m2 6M7 SWR change with rain

[Steve Katz]

That M2 link just takes you to the main website, not specifically to
anything about rain and 6m beams.  I took the liberty of copying just a few
sentences on this specific subject (pasted from M2 website directly, credit
to them!):

"CLEARLY THE CAUSE OF ANY SHIFT IS BECAUSE THE ANTENNA IS A CRITICALLY
TUNED, HIGH GAIN YAGI WITH A VERY LOW FEED IMPEDANCE. THE GAIN CURVE IN THIS
DESIGN PEAKS BETWEEN 50.200 AND 50.300 BUT IS ONLY DOWN .06 DB AT 50.000 AND
50.400 MHZ SO EVEN IF ALL THE WORKING PARAMETERS OF THE ANTENNA SHIFTED DOWN
200 KHZ, YOUR GAIN WOULD ONLY DROP BY .06 DB. AT 50.1 MHZ.

THE MOST TOUCHY SECTION IS CLEARLY THE DRIVEN ELEMENT TIPS. ANY DIELECTRIC
NEAR OR ON THE TIP OF THE ELEMENT WILL DRIVE THE FREQUENCY DOWNWARD. EVEN A
PLASTIC CAP OVER THE END WILL HAVE THAT EFFECT. WATER IS A DIELECTRIC. THE
RAIN OR WATER DROPS ALSO INCREASE THE DIAMETER OF THE ELEMENT MAKING IT GO
EVEN LOWER IN FREQUENCY. IF THE DIAMETER OF THE DRIVEN ELEMENT WERE LARGER,
WATER WOULD HAVE LESS EFFECT. WIND AREA AND COST WOULD GO UP UNFORTUNATELY.
ENGINEERING WILL LOOK INTO JUST INCREASING THE EFFECTIVE TIP DIAMETER OF THE
DRIVEN ELEMENT TO REDUCE THIS EFFECT. RESULTS OF THESE TEST WILL BE
ANNOUNCED FOR THOSE OF YOU WHO NEED HELP IN THIS AREA."

This is exactly what I said it was...rain droplets on the element tips.
When the wind blows them off or the sun dries them out, everything returns
to normal.  This is normal behavior for a narrowband yagi antenna, from what
I've seen, they all do it.

-WB2WIK/6



-----Original Message-----
From: Sadtip [mailto:sad.tip at verizon.net] 
Sent: Friday, July 27, 2007 2:57 PM
To: Hector at xe2k.net; Tower Talk
Subject: Re: [TowerTalk] m2 6M7 SWR change with rain

Hector,
	M2 put together a pretty complete description and explanation of the
problem at....

http://www.m2inc.com/index2.html

They used a 6M5X but it's still worth a read and might help.

73's
RoD
KD0XX

A good friend will come and bail you out of jail,
A TRUE friend will be sitting next to you saying.....


"DAMN THAT WAS FUN"

> -----Original Message-----
> From: towertalk-bounces at contesting.com
> [mailto:towertalk-bounces at contesting.com]On Behalf Of Hector Garcia XE2K
> Sent: Friday, July 27, 2007 4:42 PM
> To: Tower Talk
> Subject: [TowerTalk] m2 6M7 SWR change with rain
>
>
> Hi guys:
>
> A friend   just install the 6M7 in his  office tower
> and he report that his antena in the last rains the
> swr change from the perfect 50.125 1:1  to 49.9 1:1
> when the antena is WET , just dry and the SWR back  to
> 50.125
>
> I have a 6M7JHV in the box to be installed  at 150ft
> and really i will like to know if adding some clear
> spray or paint to the driven will reduce or eliminate
> that problem of SWR change when WET.
>
> any solve the problem ?  any solution  avaliable
>
> or just live with that ?
>
>
> Hector
>
> Hector Garcia XE2K / XF1K
>    Mexicali B.C  DM22fp
> P.O.Box 73
> El Centro CA 92244-0073
> USA
> http:// www.xe2k.net
> http://www.dxxe.org
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
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