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9/19/2017 7:27:10 PM
Posted: 9/5/2010 4:32:15 PM EDT
I really like the built-in antenna tuners on the modern HF rigs. However, unlike some of their external counterparts, they're all limited to approx. 3:1 SWR - None of them are equipped with the kind of L-C range necessary to tune most random wires.

So, has anyone tried an external "range extender" that could be used to add an extra 1 or 2 decades of inductance or capacitance to the internal tuner when needed? Ideally, this would be a simple, compact box that would connect between a random wire and the rig's antenna jack, and would transform the random wire's impedance to a range which could be handled by the internal tuner ( or quickly disconnected when not needed).

Thoughts?
Link Posted: 9/5/2010 5:09:46 PM EDT
"Random wires" usually require a 9:1 balun and a good ground or counterpoise.

Have you done that yet?
Link Posted: 9/5/2010 5:32:34 PM EDT
Most "random wire" antenna tuners consist of nothing more than an "L" network - No balun present.

+1 on the ground or counterpoise - It's absolutely essential.
Link Posted: 9/5/2010 7:29:48 PM EDT
I suppose the device you use to match the impedence of a random wire wouldn't necessarily be a balun - since a random wire isn't balanced. I'll have to look at the schematic of my manual tuner again, but IIRC the "random wire" connection is switched to the same point in the circuit as the center conductor of the other three coax connectors. Not the ideal random wire tuner, but most of the time it has the range and it works.

I suspect, though, if you're working against a really good ground system (as you should be), a "true" random wire tuner would be better, but that is still not a balun.

The important point is, it is not the sort of antenna system the typical internal autotuner is designed to match. Catch is, the worse the antenna system, the more likely it is it'll match to it anyway.

MFJ gets rave reveiws on their cheap manual tuners precisely because many of their users feed ultra crappy antenna systems with them. Hang a wet bedspring on the end of a 200' long chunk of 40 year old CrapShack coax working against a 12' chunk of bell wire laying on the grass and I guarantee it will be very easy to achieve 1:1 SWR on almost any frequency. Likewise, a dummy load.

So take heart! You should know, since your autotuner isn't working, your antenna system is better than that of most reviewers on Eham. And as we know, they're working all kinds of exotic DX on those wet noodles!
Link Posted: 9/5/2010 9:06:10 PM EDT

An excellent description of what type of balun to use with various antennas:

http://www.dxengineering.com/techarticlepopup.asp?ID={3E5220F7-2D0F-45B5-85F7-3B654F804C4F}

Author, Tom W8JI is the designer of a number of the Ameritron amps.
Link Posted: 9/5/2010 11:35:17 PM EDT
[Last Edit: 9/5/2010 11:36:48 PM EDT by Skibane]
Originally Posted By A_Free_Man:

An excellent description of what type of balun to use with various antennas:

http://www.dxengineering.com/techarticlepopup.asp?ID={3E5220F7-2D0F-45B5-85F7-3B654F804C4F}

Author, Tom W8JI is the designer of a number of the Ameritron amps.


When you're in the business of selling baluns, every problem looks like a nail...

Once again: The average random wire tuner doesn't use a balun - It's just an L-network, which is the same configuration used in HF radio internal antenna tuners.
Link Posted: 9/6/2010 9:24:00 AM EDT
Originally Posted By Skibane:
...every problem looks like a nail...

.


Yup. DXE even says in the aforementioned article - with an unbalanced load, the 4:1 current balun they want you to buy doesn't act as a balun per se. And how could it? Absolutely nothing in the system is balanced. A random wire against ground is essentially an end fed non resonant vertical. It is going to be fairly low impedence at odd quarter wavelengths, and fairly hight just about everywhere else, and the main issue that confronts you (besides matching) with such an antenna is common to all "imperfect" coax fed antennas - common mode current in the shield of the feedline. It is there because the RF voltage at the shield of the coax (at the antenna end) is not zero relative to ground.

But you don't have to buy anything to fix that. Just wind a choke in the feedline at the base of the antenna. The choke stops the current, and the problems of radiating feedline and RF in the shack are greatly reduced. That's not a balun. A lot of people call it that, but it is not. It's just a choke. A balun will do the same thing but it is an added insertion loss and an unnecessary expense.

Matching is another matter entirely and any tuner with sufficient range will work, IF you can tolerate the loss in the feedline between the tuner and antenna. So even if an internal auotuner (or any tuner in the shack) will present a good match to the transmitter (and it probably will if you have a really crappy ground/radial/counterpoise system), it's not going to be the best choice. Best would be to match it at the antenna feedpoint with a coupler and these days we have several choices for remote autocouplers with tremendous tuning range that do the job very well. Neither are those baluns.

You can buy a balun if you want an extra piece of mass in the system to keep warm with wasted RF energy, but you don't need one.
Link Posted: 9/6/2010 5:41:01 PM EDT
I have a mfj 910. It works great. I purchased a 909 at dayton and it never made it home. The 909 has the bypass setting in it and is great for mobile/portable use.
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