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Old 09-02-2019, 18:55   #1
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Manual antenna tuners for HF

I used to work part-time in broadcast and now am experimenting with amateur radio. I'm comfortable with manual antenna tuners, and like them. They offer certain advantages over automatic tuners, among them:
- cost
- accuracy of match
- ease of repair
- the awareness they give the operator of emergent problems with the antenna or ground/counterpoise.


They would have to be installed at the nav desk where the controls can be manipulated while transmitting. There are various strategies for connecting them to the backstay. Ladder line would be my first choice.


Has anyone done this?
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Old 10-02-2019, 06:50   #2
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Re: Manual antenna tuners for HF

I have done this. My AT-140 tuner at the stern failed, so I had to pull out an MFJ manual tuner to continue my Winlink communications. It worked fine. Even using RG-8X coax from the manual tuner to the antenna and counterpoise it worked fine. Of course, at frequencies with extremely high SWR for your antenna system you will lose some power, and some will be lost in your antenna tuner, but it won't wipe you out.

Consider this case, making calculations with the transmission line calculator included with the ARRL Antenna Handbook:

Frequency: 14 MHz
Transmission line: RG-8X
Length of transmission line: 30 feet
SWR at antenna: 20:1
Power input from radio: 100 watts

Almost a worst case scenario, right? Under these circumstances, a full 58 watts out of that 100 watts is presented at the antenna. Not even a 3 dB reduction in received signal strength at the other end.

Naturally losses would be greater at higher frequencies, but lower at lower frequencies. And an SWR that high or higher will only occur at very limited and narrow frequency ranges that can be avoided by judicious selection of antenna length in advance. If you use RG-213 instead, 71 watts makes it to the antenna. And if you can use 450 ohm ladder line (assuming you can keep it far enough away from other conductors) you will see 99 watts at the antenna.


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Old 10-02-2019, 08:05   #3
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Re: Manual antenna tuners for HF

Thanks, Chip.


It occurs to me that if ladder line were not an option due to routing difficulties, that this might be a perfect application for 1/2" (or larger) heliax. The connectors for it are expensive but it is very low loss and so even with the compounding of the loss due to high SWR it would work.


It's hard to adjust the length of an insulated backstay once it's installed, but a limited amount of matching at the feed point could reduce any trouble spots to manageable levels or at least move them to an unimportant piece of spectrum.
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Old 10-02-2019, 09:47   #4
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Re: Manual antenna tuners for HF

Some have used a 4:1 unun at the antenna feedpoint with good results, but depending on which direction you install it it might drive the impedance too low for the tuner on the 80 meter band in one direction, or too high on some bands in the other. It would depend on your tuner and the effective antenna length.

I've heard of people putting window line inside foam pipe insulation to keep it further from potential RF conductors.

Andrews Heliax LDF4-50A (1/2") diameter would deliver 85 watts in the above case. I guess if you are comfortable working with it you might find a way to do it, but I have never worked with Heliax. No idea what it would be like!

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Old 10-02-2019, 12:39   #5
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Re: Manual antenna tuners for HF

Jammer, we've been using a manual tuner exclusively for years... and we just feed the nearest chain plate and use the entire rig for an antenna. Has worked well since first attempted in 1986 when we set out (on two different boats).

My signal strength is similar to nearby boats who have insulated backstay antennae, so I not fussed about its functionality. And when we were dismasted in 1996, I was able to use a ~20 foot length of common wire laid on the deck to establish comms. Dunno if that would have worked with an AT120!

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Old 11-02-2019, 20:31   #6
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Re: Manual antenna tuners for HF

I am with SoonerSailor here. A 4:1 UNUN works at my QTH with a 35.5' wire antenna. No problem working 7MHz and 14MHz. Tunes up just fine on other frequencies as long as they are not a multiple 1/2λ which would produce a very high impedance. I would also add a choke balun before the 4:1 UNUN (transmitter side) to keep common mode currents out of the boat and equipment. Running RG8X from the rig to the base of the antenna. I think ladder line might cause issues in the boat since the maximum current will be at the rig. The ladder line would be a part of the antenna. JMHO


Out of curiosity, how long is your antenna?
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Old 11-02-2019, 20:57   #7
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Re: Manual antenna tuners for HF

Brian,


I have a 43' vertical antenna I am building at my QTH built of 2x4 sticks but the snow came before some of the parts arrived so I am awaiting warmer weather. This will have a switchable loading coil at its base for operation on 160m although I only expect very modest results. High noise environment for rx and I'm not going to try to tx at over 200 watts given that the wire is 2" away from my house. I do not expect to install HF on my Morgan 250 but anticipate a transition to a larger boat in the 37-42 foot range with a typical backstay length.
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Old 11-02-2019, 21:05   #8
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Re: Manual antenna tuners for HF

Jammer, sounds good. In lieu of "touching" the backstay, you may want to consider an alternate backstay. This would be a non-load bearing stay which will be the antenna. Use, or install, a spare halyard to raise and lower the antenna for maintenance, or to replace if need be.

I saw that you have a Morgan 250 and that has a bridge clearance of about 33 feet. The best antenna on that boat (if you were planning on that but I see you are not) would be 23'.


Anyway, good luck, sir.
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Old 12-02-2019, 05:05   #9
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Re: Manual antenna tuners for HF

Thanks, Brian.


That may be a useful exercise at some point depending on how long I stay with my present boat.
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