Primal grunt tuning

This library page:

http://part15.us/node/435 describes a tuning process yu might find useful.

Another approach - if your VOM reads AC current at the micro-amp level, would be to use the VOM in place of the light bulb in the illustration. AC micro-volts might also be useful, though likely less so.

A less desirable option is to rectify the antenna output with a diode and capacitor and use a DC millivolt VOM - I've done that and it sort of works.

In my experience, tuning is the most frustrating process imaginable because there is such a narrow band of effective match. The more variables you introduce, the worse it gets.

If both the inductor and the antenna length are tuneable, I would start with the antenna at the mid-point of adjustability and tune the coil for the highest output, then change the antenna length to fine tune that.

It just takes lots of fiddling but the reward is that the range jumps up quite a bit when you finally hit that teeny, tiny sweet spot. That has literally taken me days to find, btw 🙂

I use my trusty old GE portable AM/FM/SW receiver. It has a signal strength meter. I don't care how it's calibrated as I just need to know if the signal is stronger or weaker as I adjust the antenna.

To determine what frequency the antenna is tuned to, I simply vary my transmitter frequency up and down and watch the meter. If the maximum signal occurs lower than the desired frequency I need to shorten the antenna and vice versa. Like wise checking the meter when adjusting the loading coil taps will show up as maximum signal when you're right on.

I have an SS-Tran ground mounted with 16
radials and the "less than 3 meter antenna."
I made a different kind of tuning
coil. It has multiple taps with a ferrite bar
inside to fine tune it.

My experience went like this (the very first
time I tried to tune this system.) I had a
very inexpensive field strength meter, like
the kind you would buy in a ham radio or
CB radio store. (The kind that can also
be used as an SWR meter.)

I set the field strength meter right next to
the antenna. (I realize this can cause
interaction problems but I had no other
idea at the time.) I tried all the taps on
the coil and received a very small reading
on the meter on one tap. A "practically
nothing" reading. I was very discouraged.

At that point I got an old ferrite bar out
of a junk radio. I started to insert the
ferrite bar into the coil. Suddenly the
field strength meter jumped way up
to about half scale. That was my peak.
.
I found that my transmitted range was
good at that point. I duct taped the
ferrite bar into the coil and left it there.
From that point on, I went from a signal
that went a few hundred feet to a signal
that went a few thousand feet. (On a
good radio.)

I have also put a radio in my basement and
tuned it so my transmitting signal was nulled
to the point where is was very weak. Then
I transmitted the signal back to myself on
49 MHz with an old pair of 49 MHz walkie
talkies and listened to it while I was tuning
the antenna. The transmitting antenna, ground,
and SS-Tran are about 100 feet from my house.

This system worked fairly well, but I can see
that there could be many variables with this,
and the first idea. Still, I like to experiment
with different ideas, so it was fun for me.

That's my two cents.

Best Wishes
Bruce, MICRO1690/1700

I use my trusty old GE portable AM/FM/SW receiver. It has a signal strength meter.

May I please borrow it for a day? 😉

Seriously, I can't find one such radio around here. Where doI get one? Is it an old Superadio? Our only source here is a very small Radio Shack store. None of the radios I've seen there have a 'meter'.

As a suggestion, If you know of a local shortwave listener or ham radio operator see if they will help you with one of their general coverage receivers. They are usually sensitive enough and they have signal meters.

Find an old AM table radio, easier if it's a tube model.

If you can get a schematic or know someone that repairs the "older stuf" you can simply meter the AVC voltage from the detector. That would serve as a relative signal strength meter.

I'll try the ham radio guys, but most of them I know are 2-meter jocks. Very popular because of repeaters and portable emergency sets.

Old table radio won't do ... I'm going to use 1700 or 1710. Almost everything below those have listenable stuff / possible interference issues ... or will that not effect the detector circuit?

mram1500's suggestion is good and easy. In the old discrete component AM only radios (tube or transistor) the AVC voltage appears across the volume control pot (across the outer tabs). Unfortunately the modern radios use ICs and most of the AVC is internal to the IC though it is brought out on a pin. The problem I see by looking at the IC data sheets is that the AVC voltage change is very small with respect to the signal strength. They must have a tight control loop.

Careful if you get a hold of an "all American five" or similar AC/DC powered radio since they are not isolated from the AC power line.

It has been many years since I did this but I recall the AVC voltage was in the 1 or 2 volt range so with a DVM it would work well. It works even better if you open the AVC loop but even closed loop you get a good signal strength indication useful for tuning (though it is non-linear).

Neil

The air cap option seems like good solution if the coil is in good shape for final.

I.e., if I can get the full vertical length allowed, or just shy of it, and the coil loads it best I can get, what value range of air cap would I want for final tuning at a TX freq of, say, 1700 kHz?

Also, how come none of these little Part 15 TX' and kits come with tuning meters built in? That'd sure be a time saver when tuning up.

Take a look at the Talking House ATU schematic. It has a built in tuning meter. It consists of a microamp meter, a diode, a few resistors and caps. Looks like it could be easily duplicated.

I have the Talking House ATU and having the meter built into the unit makes tuning a snap.

Thanks ... I like 'snap' 😉 How does the TH with the ATU work for you? Is it worth the moolah?

Something I've wondered about: TH parts pictures online seem to indicate a 75-ohm cable for connecting the TX to the ATU and whip. How does that get past the transmission line length/antenna length limitations imposed by Part 15?

As I understand it, the unit was certified with the ATU as the final output, so the ATU become the point where the three meter length is measured from.

The Chez Radio ProCaster goes further than that and actully divides the transmitter into two parts, with the modulation section in the studio and the final output section on a pole outside.

Some schematics for home buit units in the Library section provide the option to have an optional remote final output and buffer amp section, too.

If that's the case, then why doesn't a loading coil represent the same sort of situation, i.e., measure the final output at the actual antenna rod end of the coil?

IOW, what if I use a loading coil instead of the ATU? Might not be certified, but it's essentially the same thing ... or isn't it?

MRAM ... how are you doing on range with the TH ATU?

Also, does anyone know of a used TH ATU for sale? I'd like to experiment, but $200 new is past my one-off pocketbook limit.

How about this thing?:
http://www.kenneke.com/antennas.html ... scroll down about 1/4 of page.

Well, actually a loading coil is an antenna tuner so to speak. That's basically all the TH ATU is as far as tuning the antenna. TH just does it a little differently by adjusting the ferrite core position which changes the coil inductance. The main difference is the packaging and features the TH ATU provides in addition to the adjustable coil inductance.

When you adjust a loading coil by either changing the number of turns on the coil or tapping different points on the coil, you are changing the coil inductance. The idea is to make the inductive reactance equal to the antennas capacitive reactance. They are opposits and cancel out when equal. It's a little more complex than that but that's the basic idea.

I suppose most think of an antenna tuner as some type of adjustable network using a coil and capacitors to not only tune the antenna but match the transmitters output impedance.

My TH ATU seems to work fairly well but that's subjective. It's definitely much easier to setup than fiddling with adjusting the number of coil turns or tap settings to tune up. I suspect that using a physically larger coil probably is more efficient resulting in better range.

I've used home made antennas with loading coils and the range was similar to using the TH ATU but much more tinkering was needed to get them going.