You only got part of the story.  TIn theory a half wave is good, but not 
not at all necessary.  A quarter wave is a no no.  Formula is wrong!!! 
The 468 comes into play for antennas because of a 5 percent end effect 
for calculating coax stubs and such, use 492 for a true half wave, and 
multiply times volocity factor, usually .70 is close enough unless it is 
foam which I wouldn't recommend anyhow.  If you really want to know what 
the swr at the antenna is, check at the feed point.  Also, put a dummy 
load at the end of the coax instead of the antenna.  I have a similar 
problem with my 160 windom antenna, nothing I've tried seems to change 
much.  Swr is good on 80, not so good on 40and so forth, but it tunes 
and works!!!
73
Butch
WA0VJR
Node 3148
Wallace, ks.


On Thu, 17 Nov 2011, Tom Behler wrote:

>    Guys:
>
> I unexpectedly had some extra time here tonight, and just got off 160 meters
> with some midwest hams, who gave me a real interesting theory about my 160
> meter sloper, the excess coax I'm using for a feed line, and why it might be
> throwing my SWR's off.
>
> Here's the theory:
>
> They said that using a 100 foot piece of coax as a feed line is never a good
> idea because there is something in that particular length that is known to
> throw antenna resonance off.  They said there was an article in QST
> documenting this fact a number of years ago.
>
> They said that what I want is either a half wave-length piece of feed line,
> or an eighth-wave length piece of coax.  They say I should avoid a
> quarter-wave piece.
>
> If you do the math (i.e. 468 over the frequency in megahertz), a half-wave
> piece of coax would be approximately 260 feet long.  This is obviously not
> very practical.
>
> However, if one were to use an eighth-wave piece, that would amount to
> approximately 65 feet long.
>
> Since I seem to have about 40 extra feet of coax out in my yard at the base
> of the antenna, cutting that off would almost give me that eighth-wave
> length.
>
> I wonder if this is really what I should now shoot for.
>
> Of course, I do not have experience soldering pl259's, so I'll have to get
> help there, but I'm really thinking this might be worth a try.
>
> You guys know far more about this stuff than I do, so if you think this
> theory is nuts, please don't hesitate to let me know.
>
> Thanks for the continuing help with this issue, and I look forward to
> working at least some of you on 160 meters around 1.845 MHZ tomorrow night.
>
> 73 from Tom Behler: KB8TYJ
>
> ----- Original Message -----
> From: "John Miller" <[log in to unmask]>
> To: <[log in to unmask]>
> Sent: Thursday, November 17, 2011 7:31 AM
> Subject: Re: SWR's continued
>
>
>> All I can say is that did happen to me the one time I coiled up about 50
>> feet of coax, it actually made it try to radiate the rest of the coax and
>> the SWR went crazy. The coax was fine, I cut off the extra and used it
>> elsewhere later on, even uncoiled and thrown all over the place it was
>> fine
>> but coiled up, it wasn't happening at all. I'm just reminded of that which
>> is why I say that.
>
>