I'll keep the list posted on what we find. I'm playing it all by the
numbers, and I don't have access to a TDR. Calculate and measure, one
step at a time is my plan. Sooner or later if we don't find a problem,
we plan to replace the Telewave F10 with another DB420 we have and if
that works well, consider a problem with the F10. I'm now wise to
looking very closely at hardline fittings, and have realized the DC
grounded antenna trick, etc.
All of the responses here have added additional emphasis or new tricks
to try -- Please keep them coming, but I'm probably going to keep them
filed and wait for some more testing for a reply on the list.
Now the thing I do want to emphasize is that Telewave has been
incredibly responsive to us and has not put unreasonable burden on us
to prove the F10 is bad before taking it back -- I have no proof of
this yet, or reason to think it's the F10, I just want everyone to
know that Telewave has been a real class act when I've called them. I
was immediately put in touch with John Hilmer, the director of their
antenna division, and he has been keeping in touch via e-mail. It is
clear that even just one pair of hams buying one antenna is important
to them.
73 for now!
Cort (N0MJS)
On Oct 17, 2008, at 12:26 PM, Nate Duehr wrote:
> Cort,
>
> It sounds like a "mechanical" problem somewhere, as others have
> mentioned.
>
> If you can get your hands on a TDR or similar device to sweep the new
> antenna and feedline on top (using calibrated 50 ohm load on the
> feedline, and then adding the antenna on for the next test), you'll
> probably find the problem.
>
> If you can't borrow that kind of gear, perhaps a jumper from the
> bottom
> feedline to the top antenna to see if it changes the performance would
> help you figure out if the problem is the new Telewave antenna, or the
> feedline going to it.
>
> Remember, the wattmeter at the transmitter will show "good SWR" if
> something is eating the RF but not radiating it. You may also want to
> consider sending someone up the tower with the wattmeter and seeing
> what's going on right at the input to the antenna, if all you have is
> the wattmeter for test gear.
>
> The key thing to remember is that the numbers should "match" theory,
> or
> be real close to it. If you're putting in X amount of power at the
> transmitter, and you know the feedline loss numbers, you can calculate
> for what you should see at the "base of the antenna" measurement. If
> the RF isn't getting there, you know it's a problem in the hardline.
> If
> it is getting there but you have high SWR at the antenna base,
> something
> is physically wrong with the new antenna, a connector, something...
>
> I think from this text and the replies of others, you can come up
> with a
> plan that will eliminate a section at a time... if you have jumpers
> inside the building to the outside, test those (a high power dummy
> load
> is nice here, if you have one that you know is a solid 50 ohm load,
> and
> not a hunk of junk that's reactive or cheap -- I like the big Bird
> dummy
> loads for this part of the "job"), test just beyond the Polyphaser (a
> friend had one show up bad from the factory lately), etc... all
> depending on how much "stuff" you have between the TX and the
> antenna...
> test at every point.
>
> Somewhere you'll find it all "falls apart" according to the numbers...
> or... if you don't, the antenna's got problems. At that point, jumper
> over to the other antenna with a nice solid piece of LMR-400 or
> better,
> and see if it behaves normally -- you'll find it!
>
> There's an art to finding this type of problem, but it's all based in
> feedline and antenna theory... if you know what the numbers SHOULD
> be at
> each test point, you're WAY ahead of the game. Make up a diagram and
> calculate feedline losses to each point, etc... it'll make it pretty
> obvious where the RF is "disappearing" if there's a
> mechanical/connection problem or a bad antenna.
>
> I'm no RF pro, but the pros I've learned from over the years would all
> tackle this problem this way... find the place where the theoretical
> RF
> behavior falls apart, and you've found the problem, most likely. It's
> better than just shotgunning in new gear until the problem is "fixed".
>
> Nate WY0X
>
>
--
Cort Buffington
H: +1-785-838-3034
M: +1-785-865-7206
------------------------------------
Yahoo! Groups Links
<*> To visit your group on the web, go to:
http://groups.yahoo.com/group/Repeater-Builder/
<*> Your email settings:
Individual Email | Traditional
<*> To change settings online go to:
http://groups.yahoo.com/group/Repeater-Builder/join
(Yahoo! ID required)
<*> To change settings via email:
mailto:[EMAIL PROTECTED]
mailto:[EMAIL PROTECTED]
<*> To unsubscribe from this group, send an email to:
[EMAIL PROTECTED]
<*> Your use of Yahoo! Groups is subject to:
http://docs.yahoo.com/info/terms/
