| Author |
Message |
    
Airplane1
Senior Member
Username: Airplane1
Post Number: 1142
Registered: 5-2004
| | Posted on Friday, December 19, 2008 - 7:12 pm: | 
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I had my swr on my maco 5/8 set good on 10 meter and a little high on 11 meters but not real bad, my tuner took care of it nice. We just got an ice storm here and since the antenna and tower are coated with ice (about 1/2 inch) the swr just went down very nice on 11 meters and I do not need my tuner. But on 10 meters the swr shot up to 2.5-3.0 or so and I want to know why this could happen.
Can it be the coax getting moist inside or can ice just do this. is it normal or indicate a problem?
Thanks
AP |
Patzerozero
Senior Member
Username: Patzerozero
Post Number: 4443
Registered: 7-2004
| | Posted on Saturday, December 20, 2008 - 7:01 pm: |
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usually with the 5/8's feedpoint design, excessive ice buildup would effectively 'shorten' the apparent length of the antenna, meaning swr would be LOWER at HIGHER freqs. i suppose it is possible the ice buildup moved the spacing on the tuning ring, showing the result you are actually seeing. if it gets warm enough for rain-like they are telling us it SHOULD eventually do here on sunday-check & see if SWR moves back to as it was. even after a few sunny days, as long as the temp is near or above freezing, check again. if it remains as it is, i'd check tuning ring for movement.
take a radio or 2, that go down below ch 1 & up to top of 10m or higher, & plot your actual swr readings & see what they do-an actual curve, or do they bounce up & down within that 3-4mhz spread.... |
Marconi
Advanced Member
Username: Marconi
Post Number: 795
Registered: 11-2001
| | Posted on Sunday, December 21, 2008 - 9:20 am: |
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Hey Pat, you may be right the ice is making the element appear thicker by maybe an 1". One inch is considerable and if the element stays the same length then the resulting center frequency will go down and not up as you suggest.
With the ice making the element appear to be about and inch thicker then the element needs to be shortened in order to regain frequency in 10 meters or the resulting center frequency will go down towards 11 meters. This is because the element appears too long---and this is what Airplane1 says is happening. |
Tech237
Moderator
Username: Tech237
Post Number: 1100
Registered: 4-2004
| | Posted on Sunday, December 21, 2008 - 9:45 am: |
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Actually a thicker conductor has a wider bandwidth and therefor could show a lower SWR at a high frequency, although Marconi is right in saying it will generally drop the frequency of resonance..
Simon
Tech237
N7AUS
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Airplane1
Senior Member
Username: Airplane1
Post Number: 1143
Registered: 5-2004
| | Posted on Sunday, December 21, 2008 - 6:26 pm: |
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Well we got 35 Degree temps and sunshine today. I hooked the MFJ259 up again this evening as I seen most of the ice is gone. The swr is pretty much back the way it was. This is good as I was hoping it was not my coax.
Now can some one tell me what the Imp.magnitude is for on this anilizer? when I switch to that setting the display shows swr and (Z)=50 at 27.860 and swr at 1.1. When I tune the freq. up or down the (Z) number moves up and down as the swr goes up.
I read the manual but dont understand this. How does this help in tuning the antenna. Should I have got a 259B instead of the plain 259 anilizer?
maybe the B version would be easyer to use.
I did not want to pass it up as it was very inexpensive and in mint condition. I wanted to learn how all this antenna tuning works so as to start an antenna farm soon, HA HA HA.
AP |
