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Author Topic: Screwdriver vs. SGC 9 ft Whip/Tuner Combo  (Read 10948 times)
KD8HMO
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« on: October 11, 2010, 10:37:22 AM »

When using a mobile HF rig in a semi truck, what would be the better antenna setup? A screwdriver antenna with or without an auto-tuner, or the SGC 9 ft whip with their auto-tuner? Cost difference aside, I would think the SGC setup would be easier to use and maintain. Any opinions?
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K0BG
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« Reply #1 on: October 11, 2010, 10:50:06 AM »

Auto-coupler/whip combination act just like a base loaded vertical. As such, the radiation resistance is 1/2 that of a center loaded one. Yes, they can work very well if you put enough ground plane under one. That rules out mirror mounts! The other issue is, for best results you need to place the couple outside the vehicle. The reason is, the actual antenna begins at the input of the coupler, not its output side. And, you cannot use coax from the coupler to the whip, period!
Doing so will greatly reduce the already low performance.

OTR truck aren't the best antenna platforms for a variety of reasons even though they're large in size. Most are plastic, and any decent chunk of metal is often to close to other parts. No matter the antenna, the most important item after the ground plane issues, is to keep the coil as far away from any material as you can.

You might want to visit my web site, and spend some time reading.
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W5DXP
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« Reply #2 on: October 11, 2010, 11:56:14 AM »

A screwdriver antenna with or without an auto-tuner, or the SGC 9 ft whip with their auto-tuner?

Both work acceptably well on 20m-10m. Seems to me the decision hinges on 75m and 40m operation.

On 75m, the 9' whip and autotuner has been measured to be 12-14 dB down from a properly functioning screwdriver. The screwdriver is about 10% efficient on 75m which makes the whip+autotuner less than 1% efficient on 75m. How do you like QRP?  Smiley
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73, Cecil, w5dxp.com
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
K0BG
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« Reply #3 on: October 11, 2010, 01:01:13 PM »

I'll disagree with you Cecil about the 80 meter efficiency of an 80 meter screwdriver. If you do everything correctly (mounting, OL length >12 feet, and a large, properly installed cap hat), you can eke out about 6%. Most 80 meter installs are closer to 1%, and a lot less than that for a hamstick.

The whole issue is about ground loss which is a minimum of 10 ohms on 80 meter, and typically double that for the average mobile.
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AA4PB
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« Reply #4 on: October 11, 2010, 01:16:05 PM »

First off, I assume we are talking about a "real" screwdriver and not one of the mini models. Second, you are right that the SGC and whip will be somewhat easier to use and maintain BUT as others have said, it will not be as efficient on the lower bands. If you are at all serious about 75M and 40M then a full-sized screwdriver is the way to go.

The SGC fiberglass whip does have a helically wound copper tape element so I assume that the efficiency will be a little better on 75M and 40M than a stainless 102-inch whip.
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KD8HMO
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« Reply #5 on: October 11, 2010, 01:23:58 PM »

My friend,KG6TOP is in the process of installing an Icom IC-7000 in his semi truck and is probably going to use the mini tarheel model. I am a semi driver too, and i plan to have the IC 7000 or a yaseu FT 857D and will need a good antenna setup too. Im just wondering how the other ham/drivers out there are dealing with this infamous ground problem that these stupid trucks have now...
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K0BG
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« Reply #6 on: October 11, 2010, 01:44:14 PM »

It depends on the truck.

Some tractors have expanded grating over the fuel tanks, and if there is adequate clearance it's about as good as you can get. Some others have metal cowls, and that can be good too. However, short, stubby antennas just do not cut the mustard. You can make contacts on a dummy load, but when you live in your truck, that get old, darn fast!

Anything simply done, relates directly to inefficiency, and there isn't a workaround, or a shortcut.
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KD8HMO
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« Reply #7 on: October 11, 2010, 04:36:18 PM »

I keep seeing references to fiberglass "marine" antennas (especially for CB radio) that require no ground. How do they make that happen and why couldnt SGC make one to use with an auto tuner? If a "marine" whip actually works, it would be a godsend to truck drivers...


Edit: I just looked at the SGC website, and they list their 9 ft whip as a "marine" antenna. it would be interesting to see this combined with their popular auto-tuner installed on a typical newer semi-truck. I would love to see SWR and radiation test results...
« Last Edit: October 11, 2010, 04:52:10 PM by Richard » Logged
KG6TOP
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« Reply #8 on: October 12, 2010, 03:23:31 AM »

My truck is a 09 international pro star, the majority of the cab and sleeper is steel along with the doors. I looked at the mirror mounts and they appear to be pretty well bonded to the all steel doors and body. The only parts that aren't metal is the hood, upper air dam, and the rear cab extenders.
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W5DXP
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« Reply #9 on: October 12, 2010, 05:28:29 AM »

I'll disagree with you Cecil about the 80 meter efficiency of an 80 meter screwdriver.

I think I actually achieved very close to 10% efficiency with a 6x14 foot hardware cloth top hat added to my HighSierra/GMC pickup installation pictured on the following web page. I squeezed another ~2 dB of measured gain out of my already top-rated antenna.

http://www.w5dxp.com/shootout.htm
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73, Cecil, w5dxp.com
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
K0BG
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« Reply #10 on: October 12, 2010, 08:36:08 AM »

The major concern is ground loss as it dominates the efficiency equation, even on 10 meters. Secondarily is resistive losses in the coil, or Q losses. They're greatly increased by any mass (metallic or not) within the field of the coil. Thus, the general rule of thumb is, to place the largest metal mass directly under the antenna, not along side, and especially near the coil.

When you mount antennas low to the ground (trail hitch mount for example), you force a major portion of the return current to flow through the lossy surface under the vehicle, rather than through the less lossy superstructure of the vehicle. A similar thing happens when you mount antennas on long stalks. A mirror mount is a good example.

In both cases, the coax carries a lot of the return current on the outside of the shield. Common mode current in other words.

All this said, you sometimes have to do what you have to do, and except whatever losses there are as a gimme. But that's not to say, you shouldn't do the best you can with the aforementioned rule of thumb.

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KD8HMO
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« Reply #11 on: October 12, 2010, 01:02:22 PM »

Wilson Antenna has come up with a fix for this problem. They now sell a 4 ft whip CB antenna with a special mount and coaxial cable. They have added an outer braiding to the cable that they say provides a good ground plane. They told me that it wont work on 10 though lol...
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K0BG
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« Reply #12 on: October 12, 2010, 01:41:45 PM »

That's so much horse pucky! That's like Firestix's 11 meter 5/8 wave vertical, wound on a 48 inch chunk of fiberglass. Or perhaps that 50,000 watt rated, 36 inch, 11 meter wonder all of the truck stops sell.
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W3LK
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« Reply #13 on: October 12, 2010, 02:57:01 PM »

Wilson Antenna has come up with a fix for this problem. They now sell a 4 ft whip CB antenna with a special mount and coaxial cable. They have added an outer braiding to the cable that they say provides a good ground plane. They told me that it wont work on 10 though lol...

(1) Some hype never dies
(2) There's one born every minute.

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KD8HMO
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« Reply #14 on: October 12, 2010, 04:42:30 PM »

If they run a braid on the outside of the cable, wouldnt that basically be the ground end of a vertical dipole? Im sure its not super-efficient, but I wonder if it would work at all? This would be a fun one to experiment with...
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