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Author Topic: Mobile antenna question  (Read 781 times)
KC4UKR
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« on: June 15, 2009, 11:08:30 AM »

I've just installed a 102" whip on my truck for mobile HF use.  The Rig is a Yaesu FT-857D with the FC-40 tuner.  The tuner is mounted underneath the bed of the truck in the rear, right next to where the antenna is mounted.  I've grounded the antenna mount to the frame as well as to the tuner.  Also, I've taken steps to minimize other noise with the muffler, exhaust pipe, hood, and spark plug wires.

All of this may be irrelevant, but can anyone provide insight as to why this will not tune 17M?  I can tune everything else from 80 to 6 (I was even surprised with the 80M tuning, even though I admit losses may still make the band unusable with this set up).  

My primary focus is 40-6, and would like 17M as well, but after the complete install, with braid straps going just about everywhere, I'm at a loss as to why the system won't tune 17?

Thanks for any thoughts/insight,

Brian
WD4DX
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WA6BJH
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« Reply #1 on: June 15, 2009, 03:43:47 PM »

The answer to the question is simple:  The FC-40 does not have sufficient matching range to adjust the impedence of the whip so that the SWR is less than 2:1.  It's just the length of the antenna and the size of the car.  If the antenna was slightly longer or slightly shorter it might tune fine.  If one of the coils in the tuner had a slightly different inductance, it might work fine.  

You can test it.  Clip a short length of wire, a meter or so long, to the top of the whip and see if it tunes on 17 meters.  If it tunes, see what bands it won't tune on with the added length.

Good luck!

73
Kerry
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WB2WIK
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« Reply #2 on: June 15, 2009, 04:40:18 PM »

102" whip on 17m is .165WL.  This will have a low resistance and a lot of capacitive reactance; the FC-40 probably just can't tune that out.

I found the SGC-230 actually *can* tune a 102" whip on all bands 7 MHz through 30 MHz, including the 18.1 MHz band (17m).   The FC-40 may not have as much range as the SGC tuner.

As suggested, you might try changing the whip length a bit.  MFJ used to make an accessory to help match HF mobile whips over a broader range, maybe that's still available.

WB2WIK/6
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K0BG
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« Reply #3 on: June 16, 2009, 05:45:30 AM »

I've had the same experience with both an AH-4 and an SGC235. In my case, the antenna was 13.5 feet in length, and made 20 meter resonant with a very wide-spaced coil. The self resonance point of the coil was well over 30 MHz. Photos of it are in the photo gallery on my web site.

Both couplers would tune the antenna, but wouldn't stay tuned. At low power levels (10 to 30 watts) everything was okay, but at higher levels the tuners would either drop out (AH-4) or retune (SG235). Whatever surface the vehicle was over also had an effect. I tried every trick I knew to fix the problem, but to no avail.

A sweep the through the 17 meter band using a borrowed Agilent VNA, revealed a secondary resonance at 16.895 MHz (no reactance, and a R value close to 50 ohms). Why it was there is perhaps debatable, but it was my thought at the time that the coupler couldn't make up it's mind which to tune; the antenna or the vehicle. This argument was reenforced when I changed vehicles, and the problem moved to 15 meters, all else (under my control) being equal. A VNA sweep of this new vehicle revealed a secondary resonance at 20.7, and another one at 22.3 MHz.

Another interesting part is, I've had about a dozen e-mails over the last few years from amateurs who have experienced the exact problem, as has this poster. Sometimes it's 20 meters, or 17, and in one case 10 meters where a CB whip should be very close to self resonance.

There may be a simple answer to the problem, but until I hear a more plausible scenario, I'll stick with the secondary resonance argument as the culprit.

Alan, KØBG
www.k0bg.com
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W5DXP
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« Reply #4 on: June 16, 2009, 06:53:10 AM »

I had a similar problem on 17m using a 13.5' whip with an SG-230 on an S-10 pickup. I finally concluded that some sort of self-resonance was occurring on 17m.
--
73, Cecil, w5dxp.com
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K0BG
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« Reply #5 on: June 16, 2009, 07:34:43 AM »

Rereading Kerry's response; if his suggestions works, I'll be VERY surprised. Been there, done that.

Apparently Cecil has had the same experience I have, and I'd bet he tried the same thing scenario, and got the same results. Nada!

Again, it a self resonant point is some other structure other than the antenna. Thus lengthening the antenna won't help. [See postscript]

By the way, we're not talking about harmonics here. Those (harmonic points) can be found with a simple antenna analyzer.

The self resonant points (discussed previously) don't always have a low SWR (close to 50 ohms resistive), but they are clearly visible on the VNA screen as +Øj.

Alan, KØBG
www.k0bg.com

PS: When I was toying with this problem (circa 2004-2005), I kept prodigious notes. One of the variables noted was the surface under the vehicle. If I parked in the vacant lot behind my house, everything seems to work well. I could even drive around the dirt lot without the problem reoccurring. As soon as I drove onto hard pavement, bingo!

If I was parked in my driveway, it never worked! I even tried adding a length of copper wire to the chassis of the vehicle. That worked once the wire was about 25 or so feet long, but driving down the road with a wire dangling behind you isn't such a good idea (even in the Alien Nation, a.k.a. Roswell, NM!).

It is also interesting to note, that John Kuecken, KE2QJ, also mentioned a secondary resonance in his ARRL Handbook treatise. Self resonance has also been noted in at least two of the Antenna Compendium series. This notwithstanding, it is not a well-known phenomena, and most certainly merits further study.
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WA3SKN
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« Reply #6 on: June 16, 2009, 11:18:28 AM »

This is one of those "Marketing Hype" versus "Reality" issues.  You will note that all antenna tuners will match "zero to infinity" impedance in the ads, later you find they meant with less than 2:1 mis-match!
You have simply found a combination that the FC-40 cannot match.  You can either add/subtract inductance or capacitance to change things and get within the range of the FC-40.
Why it won't match can be analyzed to death, and I am sure some other tuner might be able to match it but... since you already have a FC-40, I would just try another combination.
Good luck with the project!
73s.

-Mike.
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WB6BYU
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« Reply #7 on: June 16, 2009, 11:43:18 AM »

From the comments it sounds as though the body of the car
(the ground system) is half-wave resonant near the operating
frequency.  Since the impedance seen by the tuner is the
sum of the antenna plus ground impedances, this will
cause tuning problems that are independent of the length
or tuning of the mobile antenna.

Mounting the antenna in the middle of the roof should
eliminate the problem, or at least shift it in frequency,
because the vehicle resonance looks different from that
point.  You might also see a difference between a fender
mount and a bumper mount.

Even if a tuner can match the antenna under these circumstances,
the antenna likely will not work well and there may be a
lot of RF on the coax since the car body does NOT look
like a low impedance ground.
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W5DXP
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« Reply #8 on: June 16, 2009, 11:45:08 AM »

> K0BG wrote: Apparently Cecil has had the same experience I have, and I'd bet he tried the same thing scenario, and got the same results. Nada! <

Actually, the story is pretty interesting. I found that if I parked my S-10 front bumper against a metal lamp pole in the Intel parking lot, it loaded just fine on 17m and virtually turned into a beam. The antenna was mounted on the rear bumper. If I aimed the hood of the S-10 toward the receiving station, my signal climbed by at least one S-unit. The Intel security guards came out to the parking lot to find out what I was doing - circling the lamp pole and running into it at various angles. :-)
--
73, Cecil, w5dxp.com
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W0FM
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« Reply #9 on: June 16, 2009, 12:26:44 PM »

Cecil....what a HOOT!

LMAO.

Terry, WØFM
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K0BG
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« Reply #10 on: June 16, 2009, 12:49:30 PM »

Mike, in this situation, I think Dale is closer to the crux of the problem. However, based on the VNA graphing, the self resonant points appear to multiples of a 1/4 wave based on the resistive values seen.The lowest was 20 ohms resistive.

There were other self resonant points which exhibited a high resistive value, in the k ohm range, partially validating Dale's thinking.

On the first vehicle (1998 Mercury Mystique), the antenna was moved to a higher location, but that didn't help one iota.

And Cecil, I've not tried what you did, but I'm sure you've noticed reflectivity off trucks and vans as they drive by, and off steel bridge structures.

All this palaver, good, bad, and the ugly, makes for interesting discussions if nothing else. At least for me it has.

Alan, KØBG
www.k0bg.com
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W5WSS
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« Reply #11 on: June 18, 2009, 12:29:00 AM »

I encountered this exact problem on 17. I fixed the problem in less than five minutes while at my hill top location. I simply added some length to make the antenna a 17 meter quarter wave. I used some #14 copper wire wrapped it around the tip of the 102" whip added some electrical tape to hold it on Then I tied some twine to the wire end and tethered the wire with a plastic tent stake. The whip/wire combo bowed slightly back otherwise tuned perfectly as a single band antenna without the tuner or.. worked better from 20 and up with the tuner. A good solution at the time. Add a good spring to lengthen the antenna some maybe the difference will allow a better auto tune. 73
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K5END
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« Reply #12 on: June 18, 2009, 04:53:21 AM »

Have you considered using two antennas on the mobile, each with different properties, and dividing the bands between the two?

There are some pros and cons to this, one pro is an opportunity for efficiency and one con is spacing/coupling but it's worth exploring.
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N5LRZ
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« Reply #13 on: June 18, 2009, 06:13:14 AM »

Re Original Poster...

You also have to keep in mind that just because you can by configuring a coil and a cap with the proper values and in doing so can make a 6 inch piece of wire look like 50 ohms on the 160 meter band does not mean that 6 inches will transmit worth a damn compared to a full size center fed dipole (approx 250 feet of wire) properly installed at least a half wave above the ground.  

SWR is sometimes completely irrelevant.  

A dummy load gives an approximate perfect SWR but it still does not transmit worth a damn.
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K0BG
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« Reply #14 on: June 18, 2009, 06:56:37 AM »

Good point, Ray. But don't you know, that as long as you can works some DX on said dummy load, and the SWR is low, the antenna rocks!

Alan, KØBG
www.k0bg.com
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