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Author Topic: How do I trim whip to help LDG AutoTuner?  (Read 8799 times)
K7NHB
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« on: June 21, 2014, 07:02:22 PM »

I have some MFJ whips (like Ham Stick - apologies to Ham Stick) and I can trim the stinger to move the SWR "sweet spot" inside the band. I was hoping to put the whip in the band (40/30/20/...etc - different whips) and get full coverage by adding an LDG AutoTuner like the older QRP Z11 or a newer AT-100.

Would it be better to cut the stinger so it covers the high end of the band so the tuner will work to cover the low end? Or would it be better to leave the stinger long to cover the low end and have the tuner cover the high end?  Or ... I could cut the stinger so it is more in the middle and have the tuner work both sides? I'm afraid if I put it in the right middle, it won't reach either the CW area or SSB band area I'd be working (mostly QRP). In other words, I may have to choose one or the other.

Looking at an analyzer, the SWR moves up pretty fast - 5:1 towards 9:1 at the extremes, once I move out of the native stinger frequency.  Some of my rigs have built-in tuners but I think they are only good at matching 3:1 at best; I wouldn't want to push them.

The measurements above were taken when I was working with the 40M whip - maybe things are better behaved as I go to higher bands?

I attached a counterpoise for 40M and it only moved the analyzer from 1:5 to 1:4 so I'm guessing that wire will help with signal direction but maybe not so much matching. I didn't try to change the counterpoise length (coiling it to shorting it, etc.) to see if I could influence the SWR that way.  I'm thinking these MFJ whips are designed not to need a counterpoise though I never heard back from them when I emailed the question. I know they sell a bracket that holds two horizontally so you can make a single element beam out of them.

Back to my question; is the Auto tuner happier matching and antenna that is too short or one that is too long?

Thank you and 73,
Paul
« Last Edit: June 21, 2014, 07:11:04 PM by K7NHB » Logged
N8EMR
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Posts: 235




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« Reply #1 on: June 22, 2014, 05:48:34 AM »

If you have a tuner you shouldnt need to tune the whips.  DO note that the hamsticks are NOT going to work well as a  mulitband antenna even with the tuner.
YOu can get some prettty high voltages if your trying to tune up a ham stick out of band and can burn up the coils in the hamstick.

If you have a tuner then just get a whip. If you dont need 80m then a 102" cb whip will work.


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M6GOM
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« Reply #2 on: June 22, 2014, 10:22:12 AM »

If you try to use a hamstick on a band its not designed for you'll burn out the coil.
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K7NHB
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« Reply #3 on: June 22, 2014, 02:20:58 PM »

I'm sorry for the confusion. I thought this: "I was hoping to put the whip in the band (40/30/20/...etc - different whips)" made it clear I was using a different whip for each band. A tuner, especially those built into rigs, can have a limited range. As has been posted, the SWR can go up quickly when I am out of the whip sweet spot. My analyzer shows 5:1 or higher as I move from the CW portion to SSB portion on 40M.

I could keep it at the CW end and have a tuner match it at the SSB end or I could cut it for the SSB end and have the tuner match it for the CW end.  Which would be best (with and LDG tuner) was what I was asking.

73,
Paul K7NHB
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AA4PB
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« Reply #4 on: June 22, 2014, 02:50:17 PM »

Why not tune the whip for the center of the band so any frequency won't be as far off resonance?
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KD0SFY
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« Reply #5 on: June 23, 2014, 09:24:00 AM »

The AT-100 will tune up 6 to 1000 ohm loads.  If you use the 4:1 Unun, you can tune up darn near anything.  I'd adjust the antenna for the middle of the part of the band you will be using. 
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K0BG
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« Reply #6 on: June 23, 2014, 03:03:08 PM »

Well.... There are some unknowns here to deal with.

The 4:1 balun as supplied by LDG is a voltage balun, not a current balun. As a result the coupler can tune a bit broader bandwidth than it can on its own. However, this is at the expense of losses in the balun.

You have to be careful using an external coupler when matching a monoband antenna. Should you forget to change antennas, and you QSY to a band the antenna isn't resonate on, you run the risk of arcing over the coil, especially so in a hamstick-type antenna.

Someone mentioned using a CB whip instead of a hamstick. That's okay if the coupler you have can match one—the LDG cannot below about ≈17 meters, balun or no balun!

The Icom AH-4 or the Yaesu FT-40 can only match a CB whip down to 40 meters. Even then the efficiency is really low! An SGC SC230 or SG235 can match a CB whip on 80 meters, but doing so will shortly cause the tuner relays to fail!

Lastly, except for the SGC SG230 and SG235, all auto-couplers currently being sold, are strictly LC types (the SGC ones mentioned can configure themselves as Pi networks when needed). As such, matching loads near line Z (≈50Ω) is problematic. So it is best to adjust the hamsticks to their maximum overall length when fed through an LC auto-coupler. Just don't try to use one on a band it isn't tuned for!
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KA5PIU
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« Reply #7 on: June 24, 2014, 01:44:16 AM »

Hello.

It depends on what you call a "Match"
Sure, your transmitter might work into the load, but is it really radiating?
A 102" CB antenna will work for 11 and 10 meters, 6 meters and 2 meters.
But, at 10 meters it is only a 1/4 wave.
True, you can add a coil, but you are now already adding a loss of both transmitting and bandwidth.
A tuner can make a "Match" but at what loss?
Lets say you have a 3 to 1 match, and your tuner brings that down, that mismatch did not go away, the tuner is dealing with it, but with losses.
So, it is not a "Sweet spot" but just closer to resonance.
What "Freeband" CB'ers do is have a coil at or near the base.
The antenna is just a touch shorter than idea and they add or remove a tap of the coil to get it  just right.
In this manner they can go from 24 MHz to 34 MHz, a 10  MHz bandspread, on one monopole.
This is not a tuner but a real match.
The military did the same thing with 2 types of monopoles.
The VHF has this selector in the base that is motor driven with several bands from 30 to 76 MHz.
The next level up is the screwdriver antenna with the variable coil.
I stick to the monoband and taps.
10 and 12, 6 and 2 are the primary options.
You can buy or make one, nothing but a PVC pipe and a coil of solid wire with brass bolts sticking out and a tap changer switch and that sits in another larger PVC pipe for water protection.
Sounds complicated but it really is not.
You will have to build your own or buy one, there is a patent on it and it is current, so there is very little on it.
Thank you.
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K0BG
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« Reply #8 on: June 24, 2014, 02:41:20 PM »

Losses in any tuner, auto-coupler, T-match, whatever, increases as you get closer to the edges of the matching range. And, typically the losses are greater with impedances under line Z, than when matching those higher than line Z. However, you can't compare losses in a tuner, by saying losses in a loading coil are less, because that isn't the case! The matching losses in a tuner seldom exceed 15%, and are usually somewhat lower. Compare this to a mobile loading coil, where the vast majority of the power is dissipated as heat in the loading coil. On 160 meters, that's about 95% of the loss!

The real problem with mobile antennas, isn't the ability (or lack of it) to match the base impedance. After all, that can be easily dealt with, and with very low losses. Rather it is the low radiation resistance shortened antennas exhibit. In other words, efficiency losses.

There is certainly more to the subject, but in any case, we have to be careful when we talk about any kind of antenna loss, especially mobile ones.
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KA5PIU
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« Reply #9 on: June 24, 2014, 07:05:37 PM »

Hello.

This is totally correct.
An antenna that is already resonant on 10 meters needs very little added in the way of a coil, so the losses are small.
But, when your whip is small, and you are making up for it by a large coil, you lose a lot of power.
The goal should be to get as close a match as possible with the antenna.
Than use a tuner if at all possible.
In effect, a CB whip with the small coil acts more like a matching circuit than a coil.
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WN2C
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« Reply #10 on: June 30, 2014, 11:32:53 AM »

I would buy a real antenna such as a Scorpion.
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AA4PB
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« Reply #11 on: June 30, 2014, 11:54:49 AM »

Guys... The OP is not talking about using a single whip on all bands. He's wanting to use (for example) a 20M whip on 20M, using the tuner only to match the impedance at the band edges. As I said before, I'd tune the whip to resonance in the center of the portion of the band you want to use. For example of you want to work from 14.000 to 14.350 then tune the whip to 14.175. This will minimize the highest SWR encountered at the band edges and make it easier for the tuner to match. Both band edges will be closer to resonance than if you set resonance to one end and then operate at the other end of the band.
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K0BG
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« Reply #12 on: July 01, 2014, 08:17:46 AM »

Doesn't really make much difference Bob. Using an LC tuner for loads near line Z doesn't work well. The SGC 230 and 235 do okay, because they can reconfigure themselves as a Pi network. If you just have to use a tuner with a hamstick to extend the range, then make the antenna as long as possible. Even then, there may be some band/frequencies which won't match well.

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