Tuning Up Q: Open Wire + Tuner + Rig: Dummy Load??

[WBear2GCR]

Ok, so here is the question.

I use coax. So I just drop the rig on a 50ohm dummy load, tune up, switch to the ant, move a bit off frequency and tweak the output load slightly, if needed, fast.

What if you use open wire line (OWL)? One can still tune the rig into the dummy load, but the insertion of the Ant Tuner between the rig and the OWL means that another network must be dipped. Let's assume that this is needed due to a switch of bands, or whatever.

So, what's the preferred method so that long periods of dead carrier are not thrown on the band?

Is there a suitable dummy load for the output side of the Antenna Tuner to get "close"??

                  _-_-bear

[W4NEQ]

On my coaxial antenna selector switch, I include a position for dummy load.  Another position is the input of my Johnson KW matchbox which then feeds OWL for the 80/40 dipole.

This actually works better than when switching to coax-fed dipoles, because the input impedance of the tuner, once tuned, will be very close to the same 50 ohm load impedance.  Tune into the dummy, then, using a look-up table for tuner setting for the frequency, set the tuner, and switch to it.  Works great.

With a coax-fed antenna, at least for 160 /80 /40, the antenna impedance is often quite different than the load, necessitating touching up the tuning once on the antenna.

Chris

 

[Jim, W5JO]

You might consider an automatic tuner.

[W2VW]

Resettable antenna coupler, real open wire line that doesn't flake out when wet and a cheat sheet.

The 50 ohm jazz is only for instrumentation when using a tube tx. QSY and just dip the final for rated current. Put a piece of tape over the swr meter.

[W4EWH]

What if you use open wire line (OWL)? One can still tune the rig into the dummy load, but the insertion of the Ant Tuner between the rig and the OWL means that another network must be dipped. Let's assume that this is needed due to a switch of bands, or whatever.

So, what's the preferred method so that long periods of dead carrier are not thrown on the band?

It all depends on how reactive the antenna is.

In a "perfect" setup, your tuner will be connected to an antenna that's very broad, so that there's little change in reactance from one end of the band to the other. If your tuner has  logging scales on the dials, and you have a good record of the settings that work, you will be in the same (or possibly a better) place you are with coax: set the tuner, a brief tweak of the match, and you're ready to go.

If the antenna is highly reactive, you'll spend more time tuning out the reactance once you're on the air, unless you are always on the same frequency (e.g., 3885). Here again, good logging scales and records will help a lot - this is the reason so many tuners have mechanical counters and worm drives that make slight changes of the inductors and/or capacitors in the tuner easier, since they "spread" the change over a wider mechanical range by using 4/1, 8/1, or higher gear ratios.

The first order of business is to take as much variance out of the system as possible, i.e., to build an antenna that is consistently reactive by more-or-less the same amount across the band. It's good to have an antenna which is fairly broad and resonant at the center of the band segment you'll use, but of course we don't always have that choice.

The steps are the same, either way: consider "cage" dipoles or other "fat" radiators, "curtain" arrays, and other designs that are more broad. Avoid short, thin radiators (antennas that look like the whips on the backs of cars), which are always both reactive and variable.

HTH. Let us know how it works out!

73,

Bill, W1AC

[W2VW]

Oval circles on the Smith chart?

[Ralph W3GL]

Ok, so here is the question.

I use coax. So I just drop the rig on a 50ohm dummy load, tune up, switch to the ant, move a bit off frequency and tweak the output load slightly, if needed, fast.

What if you use open wire line (OWL)? One can still tune the rig into the dummy load, but the insertion of the Ant Tuner between the rig and the OWL means that another network must be dipped. Let's assume that this is needed due to a switch of bands, or whatever.

So, what's the preferred method so that long periods of dead carrier are not thrown on the band?

Is there a suitable dummy load for the output side of the Antenna Tuner to get "close"??

                  _-_-bear

So how many of you jamokes actually read the two questions Bear ask at the end of his post???

First to the last question re: dummy load, short answer:in most
cases NO

As for the first one, use an MFJ antenna analyzer in place of the
rig to tune the OWL tuner then connect the TX output coax to the
tuner (switch or patch) and your good to go...

Bear, are you aware that at least on 75, your VFO drifts a kc or
more on key-up?

[W7TFO]

[/quote]

 jamokes

Ha!

[/quote]

+1

73DG

[Pete, WA2CWA]

For all you literary people:

ja·moke  [ juh-mohk]  noun, Slang: coffee; a cup of coffee.

jamoke  noun Slang: Definition: a lowlife, a low-class person; an inconsequential person

Now if it's the 60's and you drop the "ja", definition becomes........well, I'll let you research that.

[W7TFO]

Quite simply, if you have a true balanced OWL with decent insulators feeding a center-fed dipole, the SWR is not really a problem at anything close to resonant frequency. 

It can withstand things coax would go up in flames with.

This applies only to systems with no blauns, endfed Zepps, or Marconi antennas.

Since it is operating (usually) at a high impedance, the effect on your finals is lessened.   

There is much less impedance transformation going on as with trying to get plates into 50 Ohms; the overall percentage of reflected the tubes see is less.

73DG

[WBear2GCR]

Bear, are you aware that at least on 75, your VFO drifts a kc or
more on key-up?

Ralph, that's a feature.

I've tracked it, I don't think it's a kc. It's not a kHz. either. Probably a fraction thereof.
Think the Johnson Viker Duece could use some Vreg on the VFO... it only happens when keying the rig, and not in spot mode, so likely the B+ is drifting down under load. I guess.

Thanks for the report, ur 9 by 5 too...

               _-_-bear

[Steve - K4HX]

It's not that you could not build a dummy load for this application. Two things are required:

1. You have to know the impedance at the end of your feed line and for each frequency in question
2. The R and L or C would need to be variable so as to match the numbers in 1.

Like Ralphie said, any antenna analyzer will allow you to set any tuner (OWL or coax) to the proper settings w/o radiating an interfering sig on the air. Once you log the settings for your fav freqs, you should be set.

[Ralph W3GL]

As for my resounding "NO" to the dummy load question, of course Steve is
correct.  You will note I qualified my answer with the disclaimer "in most
cases"...

However, the conditions at the antenna feed point reflecting back to
the tuner for different frequency's are so complex that I'm sorry to say,
probably less than half the current crop of licensed amateur operators are
capable of calculating the values required for the dummy load...

MFJ is also not the only source for a suitable analyzer for the tuner driver...

[W3NE]

I am presently restricted to use of narrowand antennas because of my QTH (retirement apartment). I tune the antenna at various frequencies across the band with a Matchbox adjusted for minimum SWR using an Autek RF-1 analyser. Matchbox settings are recorded so I can preset for QSY across the band. Works well, doesn't bother anybody. It is a compromise though; the 3:1 SWR bandwidth on 80M is only about 60 kc. but you do what you gotta do.

[W2VW]

However, the conditions at the antenna feed point reflecting back to
the tuner for different frequency's are so complex that I'm sorry to say,
probably less than half the current crop of licensed amateur operators are
capable of calculating the values required for the dummy load...

Used to be you couldn't get an Advanced class ticket without a grasp on this material.

Less than half?

In a generous mood?

[Ralph W3GL]

Well Dave, I decided to pull back a bit after getting pummeled for my
"jamoke" outburst...  Got several nasty-grams via PM.   But, I stand
by my original post.

Not a lot of the old class A/Advanced peoples left out there so yes.
probably a lot less than I speculated on...

It's just me, I calls em as I sees em...  But then I was never in the
making friends & influencing people business...

[W4EWH]

Not a lot of the old class A/Advanced peoples left out there so yes.
probably a lot less than I speculated on...

Sorry if people took you out of your comfort zone: it's easy to write one thing, and to have it read in a different context. FWIW, I've been there too.

I don't think theoretical knowledge is the issue here, except in the sense that asking someone how to design a dummy load for a reactive radiator is really a test of whether the other guy thinks it's a good idea.  However, I didn't think the OP was asking for one: as I read the request, he was asking how to minimize on-air-tuneup time, and (by implication) how to most effectively and economically set up a balanced radiator and a tuner so that the changeover from dummy load to antenna could be done with a minimum of tuning.

Don't get me wrong: designing a reactive dummy load would be an interesting challenge, and there are cases where they are necessary in the commercial worlds, but hams almost never have enough control over their systems to make it a practical alternative.

The problems:

• Our antennas are usually cut to fit the space and supports available, without regard to electrical factors, to rigidity (i.e., trees flexing in the wind) or to seasonal variability (snow bending the branches). That means that their reactance is highly variable and harder to predict in advance, so a dummy load would have to be able to compensate for the conditions in effect on a particular day in a particular combination of weather and operating frequency.
• Hams operator across bands that span as much as 12.5% of the upper frequency limit (80 meters), and ham antennas are therefore going to present a lot of different impedances when the operator changes frequency. Again, the dummy load would have to be changed for each frequency excursion.
• The vast majority of low band radiators are small-gauge wire dipoles, less than 1/4 wavelength above "lossy" grounds. Here we have the third strike in the ham-radio antenna playoff: thin means even more variability; by season, by weather, and by also by (of course) frequency.

All of these factors make it really difficult to predict the resistance and reactance of a typical ham antenna in advance, and thus, hard to design a dummy load to take the place of the antenna. The OP wanted a way to minimize on-air-tuneup, and I think I gave him a way to achieve it with the tools commonly available.

FWIW. YMMV.

73,

Bill, W1AC

[aa5wg]

Hi bear,

I select any band and then load the transmitter up on a dummy load.  It is now ready to go.

I now set the link antenna tuner to preset positions for that band.  I found these preset settings, for the link antenna tuner and antenna, by testing the tuner, antenna and rig together on an earlier day when the band is dead or quiet.  I did the test with low power, i.e. 10 watts.  I repeated the test at high power, i.e. 100 watts, etc.  After finding the correct settings, i.e. knob settings, circuit selection (current feed or voltage feed) I write the settings down on paper.  These settings can now be repeated quickly for each band and location within the band, i.e. cw vs. voice.

After loading up the rig I flip the antenna switch which selects the link antenna tuner.  Remember to preset your known settings to the tuner.  I apply 10 watts of power and check the settings on the tuner.  If needed I tweak the knobs while on the air.  This takes 5-10 seconds.  I repeat 5-10 second test with high power.  I am ready to go on the air.  This 10-20 second session of tests took place on a clear frequency and I ask "Is the frequency in use."  or for cw "QRL" three time and listen up and below my frequency before conducting the test.  Simple, effective and the way it's been done for decades.

I also use two small flash light incandescent bulbs.  Each side of the open wire line has one.  I don't put these bulbs in series with the line but rather I connect one end of the bulb close to where the line attaches to the tuner and clip the other end of the bulb a few inches to a foot or two away from the tuner on the same sides of the line.  This distance changes per band.  When the SWR indicator indicates a match your bulbs should be bright.  You can get a false tuned position with an swr bridge.  It is possible to get a false swr match with no lights being lit.

You want these little bulbs to be lit with a true SWR match indication.


If possible, use home brew open wire feeders.  The commercial stuff is VERY POOR and is easily de-tuned with, ice, snow, rain etc.

Chuck