[Lowfer] Beacon "IO" QSY to 185299.2

[craig wasson]

Pardon the newbie questions on the list - but I'm hoping my experience will 
help some other lurker newbies.  The transmitter is an LF-90 from North 
Country Radio - a newish offering from them that includes a built-in CW IDer 
which makes a nice convenient package with nothing needed but the 4"x4" 
board, a power source and antenna.

Looking at the schematic it seems to be a Pi network on the output side - so 
I think it would want something pretty close to the design load of 50 ohms. 
It's been the better part of half a century since I have been homebrewing 
transmitters and playing on longwave, so I'm having to re-learn a lot and 
dig out my old books.

My concern about just linking to the antenna coil with a few turn loop would 
be knowing exactly how many turns it would take to get something close to a 
50 ohm load on the TX.  This is why I was trying to ground the base of the 
coil and tap it.  My theory was that somewhere a few turns up the coil I'd 
find a 50 ohm point.  However my experience is mostly with 160M verticals 
and longwires.  Extremely short antennas seem to confuse the math.  Maybe 
that trick only works with antennas which have a feed point above 50 ohms?

The beacon is unattended while I'm working this week, so any adjustments 
will have to wait until Thursday evening.

I will be removing a section of turns from the middle of my 6" diameter coil 
to drill a hole for a shaft - I have a 4" section of PVC I plan to wind 
about 50 turns on and mount on this shaft to turn it into a variometer. 
This should let me tune more accurately, although I think I'm pretty close 
already.

I'll try simply tapping a few turns at the base for a feed point and see if 
accurate resonance helps.  Then I'll try a separate feed loop and compare 
results.  Any suggestions on how to make sure this feed loop presents 
something close to 50 ohms?  Perhaps make it resonate with the last 
capacitor on the Pi network in the transmitter?

Since I'm the ultimate lurker I don't know any local hams to help with the 
signal reporting - I'm trying to use the W5JGV argo viewer as a guide, but 
so far no hint of my signal there or on other posted images.  The HF rig in 
the car can only hear my beacon for a few hundred yards.  I know it's a 
lousy receiver at LW, but it can hear the CBE NDB for about 20 miles, so 
even with the compromise antenna and lower power I think I should hear it a 
mile or two if things were working.  I wish the W3EEE argo viewer was still 
working since that was a lot closer to me.

I can take this discussion off list if that is preferred, but like I said - 
there may be others who have been lurking for a long time and need this kind 
of info to dive in.  I've scoured the available online documents, but there 
is a gap between very limited newbie info and the advanced fine-tuning tips 
that assume you know what you are doing.  Maybe I'll take the lessons I 
learn and write a how-to guide.

Thanks!

Craig



----- Original Message ----- 
From: <jrusgrove at comcast.net>
To: <craig at wasson.com>
Sent: Sunday, February 07, 2010 5:31 PM
Subject: Re: [Lowfer] Beacon "IO" QSY to 185299.2


> Craig
>
> What are you using for a final amplifier?
>
> If it is the 'standard' Class D totem pole like LEK and TAG have described 
> it will operate into a wide range of impedances. In that case there is no 
> need to ground the bottom of the loading coil to create an auto 
> transformer to match. Basically you let the antenna overall resistance end 
> up where it will and adjust the voltage to the final amplifier for 1 watt. 
> You'll need a variable voltage regulated supply. As you bring the voltage 
> up the current will rise as well. Just stop when the voltage times the 
> current equals 1 watt. These finals are > 95% efficient so you can figure 
> your dc input power is essentially your rf output power.
>
> You'll almost certainly need a variometer to bring it to resonance. With a 
> typical Q of 300 or more you're not likely to hit resonance without an 
> adjustable element. RF current should probably be in the 100 - 150 mA 
> range. Don't worry about a clip lead rag tag setup at these frequencies - 
> you can clean it up once you get it working.
>
> Let me know if I can be of further help.
>
> Jay W1VD  WD2XNS  WE2XGR/2