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Author Topic: ONLY 40 meters for a while  (Read 2596 times)
W1VT
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Posts: 904




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« Reply #15 on: January 01, 2013, 07:12:38 AM »

The popularity of the 43 foot length stems from the fact that it does have a really high impedance on any of the ham bands, so loads up well. In other words, a good compromise.  My 20ft flagpole works well on 40 through 10M.

Zack W1VT



Did you mean to write it does NOT have a high impedance on any of the ham bands?

73,

Chris/NU1O
Yes, that is what I meant.  Due to the harmonic relationship of the popular  ham bands, a resonant low impedance quarterwave will be a undesirable  high impedance on even harmonics.

Zack
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W1VT
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Posts: 904




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« Reply #16 on: January 01, 2013, 07:20:59 AM »


Zack, the Balun Designs UnUN is rated at 5Kw, so I really dont think 500 watts would bother it under any conditions. The obvious problem is the moisture in the UnUn, and I dont know how to keep that from happening again. I feel sure its condensation, even tho I drilled a couple "weep holes" in the bottom corners, per the instructions. Gets pretty damp and muggy here in Florida, pretty well year round. I have dried it out and am going to put it back and see what happens shortly.
73, Gene AF3Y

Broadband baluns typically have a moderate choking impedance, typically between 1000 to 2000 ohms--if you have an antenna analyzer you can measure it by measuring the impedance between ground and one of the output  terminals.  The impedance of a half wave vertical is also 1000 to 2000  ohms, which can also be measured.  As a rough approximation, half of your 500 watts is lost in the balun.  5kW only applies to the ideal situation, where perhaps 10 watts is lost in the balun--ferrites can be very efficient in ideal circumstances.  But, ferrites are poor conductors of heat, which means that ferrite baluns burn up easily.  SWR  drift during transmit is a very bad sign.  Sad  

Zack W1VT
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WB3BEL
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Posts: 97




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« Reply #17 on: January 01, 2013, 07:37:00 AM »

I find that if an antenna is deaf on RX and heals temporarily post TX, that there is most likely a high resistance connection somewhere in the antenna.  What happens when you transmit is that the RF current makes a lower resistance connection like an arc welder that may "fix" the antenna for a short while until the high resistance returns.  But this temporary arc weld may still make for a poor connection and it may get worse with time.  Look at your coax feeder connection or balun to coax or balun to radiator connections or a bad PL259 to coax joint etc.  Look for corrosion and clean all the connections.  If its a multiband check the connections of traps or stubs or loading wires.

I find that most ferrite baluns are relatively unfazed by dampness or slight water at power levels up to 600-700W.  If they are sitting in a pool of water it would  be a problem obviously.  I don't think the balun is your problem unless it has an intermittent high resistance electrical connection.
« Last Edit: January 01, 2013, 07:41:17 AM by WB3BEL » Logged
AF3Y
Member

Posts: 3882




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« Reply #18 on: January 01, 2013, 07:40:04 AM »


Broadband baluns typically have a moderate choking impedance, typically between 1000 to 2000 ohms--if you have an antenna analyzer you can measure it by measuring the impedance between ground and one of the output  terminals.  The impedance of a half wave vertical is also 1000 to 2000  ohms, which can also be measured.  As a rough approximation, half of your 500 watts is lost in the balun.  5kW only applies to the ideal situation, where perhaps 10 watts is lost in the balun--ferrites can be very efficient in ideal circumstances.  But, ferrites are poor conductors of heat, which means that ferrite baluns burn up easily.  SWR  drift during transmit is a very bad sign.  Sad  

Zack W1VT

Just put it back in line after drying it out.... Nope, its DOA.  However, nothing appears to have bent burnt inside, no smell, etc. Took it back out, so I am back on 40 meters. hi hi

I am going to send it to Balun Designs and ask them to take a look at it and tell me why it is kaput.

73, Gene AF3y
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W1VT
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Posts: 904




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« Reply #19 on: January 02, 2013, 03:59:47 AM »

There is an obvious solution to balun overheating--air cooling  Grin

Seriously, it works for computers and automobiles.  Most guys know that if the air cooling of a car stops and you continue to drive--your engine will overheat. 
Even my HF has a fan.

Instead of sealing up a balun in a tiny package where it can't get rid of heat, just put it in a decorative bird house or dog house for weather protection, and install a cooling fan.

Zack W1VT
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AF3Y
Member

Posts: 3882




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« Reply #20 on: January 04, 2013, 01:43:18 PM »

New UnUn in place, all is well, back on all bands now (6 thru 80, that is Roll Eyes). Sending the bad UnUn to Balun Designs.  Bob thinks its the connector (Not mine, the connector on the balun box).

Now, bring on some DX!!   Grin Grin

(Just worked JD1BMH on 30m Wink. Nothing new, but the station IS working fine!)

73, Gene AF3Y
« Last Edit: January 04, 2013, 01:53:17 PM by AF3Y » Logged
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