[AMRadio] Antenna tuner info


Geoff/W5OMR ars.w5omr at gmail.com
Wed Jan 10 10:08:29 EST 2007


Brett gazdzinski wrote:
>  
> As I think about building a tuner, I was 
> wondering what size components would be needed
> for my power levels.
>   

Make everything capable of handling your maximum output, x2.

ie: if the B+ on a capacitor in a modulated stage should be 2500v, use 
one rated for 5kV.

> The pair of 813's can run up to 700 watts
> of carrier output, but I usually turn it down
> to about 500 watts out, around 1500 watts pep.
>   

You do realize that this is less than 100% of audio, correct?  
100w of carrier, modulated to 100% = 400w PEP

> the antenna will likely be about 100 feet
> in a flat top setup, up about 30 feet, fed with 
> about 30 feet of open wire line, made with #12
> wire spaced an inch or two apart.
>   

John/WA5BXO built an antenna for 'portable' use, which is about as you 
are describing.  Except, his feedline is a pair of 12ga stranded lines, 
spaced around 4 or 5 inches apart.  Being in the Computer Repair and 
service business, he had access to 'boxes' full of drive-bay covers.  
So, whatever that distance is, is about where his feedline is spaced. 
5.25"?  Those drive-bay covers also make it easier to roll up the feed 
line, but experience has proven over time that they are just as subject 
to the rigors of UV light.


> Cardwell is selling new caps, model 153-11-1 100pf 7000 volt
> for about $100.00 (for a shiny new cap).
>   

Man... couldn't you take some older oil-filled caps, and series/parallel 
'em to get the same value at about $5.oo (or less) per at a Hamfest?

> Would 7000 volts be enough for most situations?
> What size copper wire to make the coil with?
> Bigger is likely better, but what is the minimum
> size that would work, #12, #10?
>   

If it's available, Brett, I'd be tempted to use 8ga solid copper wire. 

> Does anyone know what sort of parts were used in the
> Johnson KW matchbox?
>
> The ARRL handbook says for 250 watts of AM, 4500 volt
> caps and #14 wire are good.
>   

You know, lots of tuners were made by simply tapping an air-dux coil.  
Whatever the size of most of those, then, should be sufficient.

> Or, what do you guys use for GOOD wire and rope?
> The rope is in tree's, and the wind rubs it some,
> the best stuff from home depot holds up 4 or 5 years
> before it looks bad enough to change out.
> Anything better out there?
>   

How about a pulley, tied to a tree, and the end of the rope pulled 
through the pulley, with a weight on the end of it?  Let the tree sway - 
the rope will roll back and forth through the pulley and not become 
frayed by rubbing on the tree.

As far as antenna wire, any conductor will work.  I've had fairly good 
luck with simple 10 or 12ga copper wire.  Of course, the price of copper 
has gone sky-high.  A 500' roll of 12ga insulated copper wire at the 
hardware store used to cost $25.oo.  If you want something a little 
larger in size (for a larger surface area - think 'skin effect') guy 
line is a conductor, but it's hard to work with.  You would probably 
have to use cable clamps and some conductive grease would help stave off 
corrosion over the years.  Guy wire is galvanized and holds up towers 
for years and years and years.


Hope that helps, Brett...

--
73, etc
-Geoff/W5OMR




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