K3 power reduction due to high SWR

[vk4tux]

You have some serious performance limitations with this setup.
If I have to use your antenna, then my move would be to replace the 250' of
75 ohm coax with 250' of 450 ohm ladder line, check the fan dipole legs are
exactly the same to provide a balanced load.

Get a balun  and short 50 ohm coax section to get from the ladder line back
into the shack, if you can't get the balanced line in to a balanced line
tuner etc.

As it is you are losing 57% @ 20m of your power in that feedline.

A mfj-986 or similar with balanced line is probably the go on a limited
budget.

You will reduce loss from 57% to 9%.

Adrian ... vk4tux

-----Original Message-----
From: [hidden email]
[mailto:[hidden email]] On Behalf Of David Stratton
Sent: Thursday, 5 April 2012 10:34 AM
To: Elecraft Reflector
Subject: [Elecraft] K3 power reduction due to high SWR

At what point does the SWR have to rise up to before

the K3/10 output power begins to decrease?

I have been using a TS-830 which is not bothered by

SWR up to at least 3:1. I cannot get the tuner now so

will I be forced to run very low power if my SWR is 2

or 3 to 1? What about the K3/100?

 

My antennas are resonant fan dipoles (80, 40, 20, 17)

fed with 250 feet of 72 ohm cable TV coax.

 

Dave - KO4KL

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[KU7Y]

It all depends on just what he means by 72 ohm cable TV coax.

If it's the low quality coax used around your TV's, you might be right.

But if it's the 1/2" CATV hardline or the smaller "drop" cable, the loss
will be much less.

These last two are quite good and have very low loss.  I have used just
under 300' and did quite well with both QRP and QRPp.

It used to be that you could go to your local cable TV company and get lots
of left over pieces of that good coax.  No idea how that works now but sure
worth a try.

So Dave, I'd say to use whatever you have and get on the air.  Then spend
the rest of your life making your antennas bigger, higher and better!

OK, back in my hole,

Ron, KU7Y
SOWP 5545M
Arizona Outlaws Contest Club
Brenda, AZ (Winter)
Caldwell, ID (Summer)
[hidden email]
http://www.hatpinsandmore.com
----- Original Message -----
From: "Adrian" <[hidden email]>
To: <[hidden email]>
Sent: 04 April, 2012 8:00 PM
Subject: [Elecraft] K3 power reduction due to high SWR

> You have some serious performance limitations with this setup.
> If I have to use your antenna, then my move would be to replace the 250'
> of
> 75 ohm coax with 250' of 450 ohm ladder line, check the fan dipole legs
> are
> exactly the same to provide a balanced load.
>
> Get a balun  and short 50 ohm coax section to get from the ladder line
> back
> into the shack, if you can't get the balanced line in to a balanced line
> tuner etc.
>
> As it is you are losing 57% @ 20m of your power in that feedline.
>
> A mfj-986 or similar with balanced line is probably the go on a limited
> budget.
>
> You will reduce loss from 57% to 9%.
>
> Adrian ... vk4tux
>
> -----Original Message-----
> From: [hidden email]
> [mailto:[hidden email]] On Behalf Of David Stratton
> Sent: Thursday, 5 April 2012 10:34 AM
> To: Elecraft Reflector
> Subject: [Elecraft] K3 power reduction due to high SWR
>
> At what point does the SWR have to rise up to before
>
> the K3/10 output power begins to decrease?
>
> I have been using a TS-830 which is not bothered by
>
> SWR up to at least 3:1. I cannot get the tuner now so
>
> will I be forced to run very low power if my SWR is 2
>
> or 3 to 1? What about the K3/100?
>
>
>
> My antennas are resonant fan dipoles (80, 40, 20, 17)
>
> fed with 250 feet of 72 ohm cable TV coax.
>
>
>
> Dave - KO4KL
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html 

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[Jim Brown-10]

> If I have to use your antenna, then my move would be to replace the 250'
> of75 ohm coax with 250' of 450 ohm ladder line, check the fan dipole legs
> are  exactly the same to provide a balanced load.

You're on target about loss in the feedline, but ladder line is not a
good move here.David has resonant dipoles, so they are a pretty good
match to 50 or 75 ohm cable. Folks think QRP, so they think small coax,  
But when the line is as long as these are, the loss certainly gets out
of hand unless it's  bigger coax. If I were to make ANY change in the
feedline, it would be to a decent RG8 or RG11.  I would also look at
ways to reduce the length of the line by finding a shorter route to the
shack.  You don't need expensive coax -- Davis 213 is about as good as
it gets for the HF bands, and it's fairly inexpensive.

Window line is NOT the cure for all ills  -- it is as lossy as small
coax when it gets wet, and there's some loss (and cost) in any impedance
transforming balun. Further, most real antennas are unbalanced by their
surroundings, so they need a good ferrite common mode choke at the
feedpoint whether they're fed by coax or parallel wire line.

73, Jim Brown K9YC
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[Barry]

I use a fan dipole as well (40, 20 and 10), and IIRC from modelling it, putting a 17m wire in the mix really messed up the match.  You might want to try removing the 17m dipole and see if there's a better match.

Barry W2UP