[Lowfer] UWL station pics and measurements

jrusgrove at comcast.net jrusgrove at comcast.net
Sat Feb 12 18:19:08 EST 2011 

Eric

Glad to hear you still have the R-75 ... it's a great little receiver.

While there is no specific size for the variometer coil,  I use a ratio of about 4:1 or 5:1 (main 
coil turns: moveable coil turns). This ratio and the smaller form diameter of the moveable coil 
seems to work out about right. Too much L in the moveable coil and the adjustment is touchy ... too 
small and there isn't enough adjustment range for varying conditions. Compromise is the name of the 
game.

There are a number of ways to couple power to the antenna but perhaps the three most popular methods 
are 1) tapping the main coil as you are doing, 2) using a coupling coil as JB is doing and 3) using 
a ferrite transformer at the bottom of the loading coil which is what I'm doing. Each method has 
advantages and disadvantages but the end result is the same - power being coupled to the antenna 
system.

To make the discussion a bit simpler let's look at JB's 'coupling coil' system first. Keep in mind 
that resonating the antenna and coupling to it are two separate items (although there may be some 
interaction). The antenna is brought to resonance by selecting an appropriate amount of main coil L 
(using taps as a coarse L adjustment) in combination with the moveable variometer coil (fine L 
adjustment). The coupling coil wrapped around the main coil is nothing more than a primary and 
secondary of a transformer. The turns ratio between the primary and the secondary establishes the 
impedance that's seen by the transmitter. The turns ratio can be worked out mathematically but we'd 
need to know the loading coil inductance and the antenna resistance. Without that information, but 
considering a typical 185 kHz antenna/loading coil, starting with 3 - 6 turns would be reasonable.

Next, a quick word about the push pull, Class D switching transmitter that many stations are using. 
Unlike radio equipment most folks are familiar with, it doesn't need to run off 12 volts - 
necessarily. It can ... but it doesn't have to. For lowfer work it's advantageous to power the 
output stage of the transmitter from an adjustable supply of maybe 10 - 30 volts. The adjustable 
supply allows us to set the transmitter for 1 watt output into a wide range of load impedances.

Once the antenna is at resonance (by performing the receiver test) it's time to connect the 
transmitter to the coupling coil. An rf ammeter, either commercial or homebrew, is inserted between 
the loading coil and the antenna. Key the transmitter using a low power supply voltage and peak the 
variometer for maximum current on the rf ammeter. Bring up the transmitter voltage until the power 
supply E X I = 1.0 (watt). Check that the antenna is still at resonance with the variometer / rf 
ammeter. That's all there is to it.

If you've guessed correctly on the number of coupling coil turns you'll have reasonable E and I 
values for 1 watt transmitter power ... maybe in the range of 12 volts @ 83 mA to 24 volts @ 42 mA. 
If it's not in this range, or you have a set dc voltage you would like to use, simply adjust the 
number of turns on the coupling loop, readjust the voltage and calculate the power. After any change 
to the system always recheck resonance.

Eric ... you're system with the transmission line tapped on the main loading coil is very similar. 
Instead of a separate primary and secondary transformer winding both windings are connected together 
as an autotransformer. The number of turns between the coax shield (coil ground) and center 
conductor is the transformer primary and the total number of turns on the loading coil is the 
transformer secondary.

Suggest anyone interested in an in depth look at these antenna topics to check out ON6YD's excellent 
antenna write-up at  http://www.strobbe.eu/on7yd/136ant/ . It's well worth the read.

Jay





















----- Original Message ----- 
From: "Eric Smith" <esmithmail at gmail.com>
To: "Discussion of the Lowfer (US, European, &amp,UK) and MedFer bands" <lowfer at mailman.qth.net>
Sent: Saturday, February 12, 2011 12:36 PM
Subject: Re: [Lowfer] UWL station pics and measurements


> 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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