[Lowfer] UWL station pics and measurements

[Eric Smith]

Jay,

THANK YOU for your help.  Yes, I do have an R-75 .. you certainly gave me
some ideas about what to do this weekend.  Hopefully I can make some
progress.

Excellent idea on using the R-75 to find resonance.  I'm sure I've seen
suggestions like that to others on here before, but it never "clicked" with
me.

And yes, I used a variometer before, both on UWL and also when running XFX.
But I never could figure out what was going on when I made adjustments.  So
I ditched it.  Now, with your instructions below, I think I can do it!
Question -- how "large" (in terms of L) should the variometer be?  Some
percent of the main coil?

Regarding coupling -- ok, I'm going to reveal my ignorance again here:  the
braid is connected to the cold end of the loading coil along with my 2"
copper strap which goes to a standard 8' x 5/8" ground rod driven into the
ground there.  The center conductor clips to my chosen tap, also not real
far from the cold end.

The antenna is just connected to the extreme hot end of the loading coil.
That is, the top end of the coil wire is connected to the antenna wire.

Years ago, before MP went to a loop, I saw where he was using a few turns at
the bottom and let that inductively couple into his loading coil.  Seems
like I tried that back when running XFX at 200 watts, but I must have just
gotten lucky ... it worked, and I never knew just exactly what should be
adjusted this way or that.

Question about that -- how many turns does a coupling loop like that need to
be?  And I was amazed by the fact that this seems like a DC short circuit
... the loop essentially ties the braid to the center conductor.  But, I do
know that inside my final there is a capacitor in series with the output,
just before the filter, so I suppose there can really be no DC short
circuit.  But there is at AC -- can you explain this?  I'm assuming it has
something to do with the number of turns in the coupling loop and the
frequency of operation ... ?

Thanks so much for your help.  Good for me to finally get an understanding
of this before I build one of your 500 watt bricks to put on 600m :)

Eric KD5UWL / WD2XFX


On Fri, Feb 11, 2011 at 8:28 AM, <jrusgrove at comcast.net> wrote:

> No slings and arrows ... just constructive comments. You probably know most
> of the following already
> ...
>
> While it may be possible to get lucky and hit resonance with a few taps and
> no moveable variometer
> coil it's not likely (at least I've never been that lucky). A decent
> loading coil will be of high
> enough Q that tuning will be very sharp. One would have to hit the correct
> tap position - probably
> within an 1/8 of a turn or less! That's one reason why there needs to be
> some form of adjustment.
> Also, water, ice and changes in the immediate environment will shift the
> resonant point. Since the
> moveable variometer coil is much smaller than the main coil it provides the
> fine adjustment that's
> needed. I'd suggest reinstalling the one you have.
>
> For tuning, start with the variometer at a 45 degree angle (coils series
> aiding - not opposing) and
> check for resonance using one of your existing fixed taps. One simple and
> quick way to find
> resonance is to replace the transmitter with a general coverage receiver
> (IIRC you have an R-75?)
> and spin the dial noticing at what frequency the background noise peaks.
> That's resonance. By
> observing the resonant frequency using other existing taps on the main coil
> you should be able to
> predict (roughly) the correct position of the tap for your desired transmit
> frequency. The correct
> tap point is where the receiver noise peaks at your desired frequency and
> the moveable variometer
> coil is at roughly 45 degrees (series aiding - not opposing).
>
> Once the antenna is at resonance the next step is to couple power to the
> antenna. It looks like
> there might be a link coil wrapped around the bottom of the main coil ...
> if not, what method of
> feeding power to the antenna are you using?
>
> Jay
>
>
>
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