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Author Topic: HF Mobile Antenna Question  (Read 8545 times)
K0JMS
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Posts: 6




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« on: February 18, 2011, 02:38:03 PM »

I'm interested of using a ball mount, 102" whip, and tuner for HF mobile installation on a ford ranger.  Any thoughts on performance from 80 - 10 meters?

Thanks and 73,
John
K0JMS
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W5DXP
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« Reply #1 on: February 18, 2011, 03:39:55 PM »

Here are some 75m mobile antenna shootout results:

http://www.w5dxp.com/shootout.htm

The CB whip with autotuner is estimated to be 14 dB down from a good 75m mobile antenna which might be 10% efficient. That means for every 100 watts delivered to the tuner, 0.4 watts might be radiated. If you adopt that configuration, I hope you appreciate the challenge of SSB mini-QRP operation on 75m mobile.
--
73, Cecil, w5dxp.com
« Last Edit: February 18, 2011, 03:41:54 PM by W5DXP » Logged

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.
WD5GWY
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Posts: 393




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« Reply #2 on: February 19, 2011, 04:18:59 PM »

I have used a 102" whip and auto tuner in an 18 wheeler for HF. It works ok on 20 -10 meters
and mediocre on on 40 meters(and not at all on 80 meters). Before I got my Alpine Screwdriver antenna for my Dodge pickup,
I used the same setup, auto tuner and 102" whip. The screwdriver antenna beats the pants off
of the auto tuner and whip combo.  I have a Little Tarheel II antenna that I have setup and use
now on the 18 wheeler, and it too , even though it is not the best of screwdriver antennas(since it is a small antenna), beats the auto tuner/whip combo.
 But, if the auto tuner/whip combo is all you can afford, then it is better than nothing. Or even use
Hamstick antennas instead of the auto tuner and whip. (I've done that too, and it appears to me that
in most cases ...especially on 40 meters that the hamstick antenna I used was better......slightly)
Good luck and hope you get something going. Mobile HF is fun and addictive!!
james
WD5GWY
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KE0NW
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Posts: 7




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« Reply #3 on: February 19, 2011, 07:26:11 PM »

Here is a link to a great site, lots of reading but very good information on mobile installations and theory in layman's terms.

http://www.k0bg.com/
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N5UD
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Posts: 800




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« Reply #4 on: February 19, 2011, 08:06:16 PM »

Forget about working any band below 20 meters with just a whip and tuner. In a few years maybe we won't need 40 meters as sunspots increase. But today, get a screwdriver.

Dave
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K0JMS
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« Reply #5 on: February 20, 2011, 07:08:49 AM »

Thanks for the input and thoughts.

73, K0JMS
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WX7G
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Posts: 5982




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« Reply #6 on: February 21, 2011, 08:38:22 AM »

Here are some 75m mobile antenna shootout results:

http://www.w5dxp.com/shootout.htm

The CB whip with autotuner is estimated to be 14 dB down from a good 75m mobile antenna which might be 10% efficient. That means for every 100 watts delivered to the tuner, 0.4 watts might be radiated. If you adopt that configuration, I hope you appreciate the challenge of SSB mini-QRP operation on 75m mobile.
--
73, Cecil, w5dxp.com

The 102" CB whip with SGC-230 autotuner gain is ESTIMATED and should not be cited as a valid reference point.

What is a screwdriver antenna? A whip with a base-to-middle loading coil. What is a 102" CB whip and tuner? A whip with a base loading coil. What's the difference but loading coil loss. We are told that the tuner loading is very, very lossy. But, if the tuner were to consume virtually all of the power it would burn up at 100 watts. But it doesn't burn up and this tells us the tuner isn't as lossy as we might be led to believe.  

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WA6BJH
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« Reply #7 on: February 21, 2011, 09:41:50 AM »

I've seen references to these shootout things before and I don't understand what they mean by "15 dB down."  I even saw one for "25 dB down."  Setting aside all the concerns about how it was measured, what does 15 dB down mean?  Ten dB is one order of magnitude and then 3 dB doubles and 2 dB slightly less than doubles.  So, if I changed from a CB whip with an autotuner to a screwdriver with a top hat, it would be same as keeping the CB whip and tuner but increasing my power from 100 watts to something over 2,500 watts.  That would be a rather outrageous claim, so whoever is making these claims must mean something else.  Please tell us what you mean.

We should all remember that mobile antennas tend to be inefficient, but I doubt that one is 15 dB better than another.

So, Give the CB whip and tuner a try. If it works for you, use it.

73
Kerry
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WX7G
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« Reply #8 on: February 21, 2011, 12:35:54 PM »

15 dB down means the signal from antenna X is 15 dB less than the signal from antenna Y.

Most antenna shoot out data I've seen is worst than no data. Bad data =  bad decisions.
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W5DXP
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« Reply #9 on: February 21, 2011, 12:58:13 PM »

The 102" CB whip with SGC-230 autotuner gain is ESTIMATED and should not be cited as a valid reference point.

Well, a 138" whip plus SG-230 was measured to be 12 dB down from the top-rated antennas on 75m. A 138" whip has a radiation resistance of about one ohm. A 102" whip has a radiation resistance of about 0.56 ohms. Why would the 102" whip not be ~2 dB down from the 138" whip?

Quote
What is a 102" CB whip and tuner? A whip with a base loading coil. What's the difference but loading coil loss.

The SG-230 iron powder inductances are capable of dissipating a lot of heat. They are inefficient because there are a bunch of them switched in series. Compared to a high-Q air-core loading coil, there is no contest as proved by the shootout results. An SG-230 driving an 138" whip was 7 dB down from a 102" whip with a Texas Bugcatcher base loading coil. The coils inside the SG-230 do not radiate (much). The Texas Bugcatcher coil radiates quite a bit because it is out in the open at the highest current section of the antenna. One wonders what the result would be with the Texas Bugcatcher coil located inside a car trunk.

On my same GMC pickup, the SG-230 plus 138" whip was 12 dB down from my top-rated antenna.

Quote
But, if the tuner were to consume virtually all of the power it would burn up at 100 watts. But it doesn't burn up and this tells us the tuner isn't as lossy as we might be led to believe.

Bad premise and therefore a false conclusion. The SG-230 uses husky #2 powdered iron cores. I have accidentally loaded a four foot Hustler bottom section on 75m because I forgot to install the loading coil and top section. The SG-230 did NOT burn up and seemed perfectly capable of sinking 100 watts.
--
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.
W5DXP
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« Reply #10 on: February 21, 2011, 01:30:28 PM »

Setting aside all the concerns about how it was measured, what does 15 dB down mean?

Comparing a test antenna to an ideal antenna with an efficiency of 100%, 1dB down = 79% efficiency, 3 dB down = 50% efficiency, 5 dB down = 32% efficiency, 10 dB down = 10% efficiency, 15 dB down = 3% efficiency.

Comparing a test antenna to an antenna with a known efficiency of 10%, 15dB down = 0.3% efficiency. Question is: Where did the 15 dB down figure come from? The worst of my reported measurements is only 12 dB down, i.e. 2 standard S-units.

Is it such a stretch to realize that a CB whip plus SG-230 autotuner is two standard S-units down from the best of the best 75m mobile antennas? If one were to listen to two mobile rigs parked side by side with one coming in at 12 dB over S9 and the other only at S9, why would one be surprised? It happens all the time.

Quote
So, if I changed from a CB whip with an autotuner to a screwdriver with a top hat, it would be same as keeping the CB whip and tuner but increasing my power from 100 watts to something over 2,500 watts.  That would be a rather outrageous claim, so whoever is making these claims must mean something else.

Not outrageous at all. Let's say the reference antenna is 10% efficient. With 100 watts incident, it would be dissipating 90 watts and radiating 10 watts. With 2500 watts incident, it would be dissipating 2250 watts and radiating 250 watts. It would need a very husky loading coil. That's why antennas have power ratings.

Quote
We should all remember that mobile antennas tend to be inefficient, but I doubt that one is 15 dB better than another.

Everything I have posted here is based on actual measurements performed by competent RF engineers. The results were consistent over three separate shootouts over as many years. Just because they go against some people's gut feelings is of no technical consequence.
--
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 #11 on: February 21, 2011, 02:53:06 PM »

There is one item here which needs a bit of clarification.

Auto-couplers (call them antenna tuners if you please) can get rather lossy, especially at the edges of their matching capabilities. One common reference is an 8 foot CB whip driven with an AH-4. Seemingly, the combination works well, and folks make contacts with the set up. Who cares if it is a couple of S units down from your average screwdriver?

Typically, nothing bad happens if you're cognoscente of the high RF voltage present, even on 40 meters where it is at its max matching range. But, how many folks have put their hand atop their AH-4 after a long QSO? I suspect not many. If they did, they'd easily feel the warmth. I personally have never seen one with a melted cover, although I've been told by a few users that their's did.

The same type of losses can happen to even large base stations tuners when running modest power when they're improperly tuned and/or at their matching limit. They don't melt perhaps, but they sure do arc!

The only reason some mobile antennas don't burn up, is simply due to the fact they have enough heat dissipation; surface area if you will.

The whole point here is, you can work the world with modest power if conditions are correct. The only addition you'll get if you go all-out with your mobile installation, is a few more contacts, you might not make otherwise.
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WA6BJH
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« Reply #12 on: February 21, 2011, 06:57:22 PM »

How were the measurements taken?

What were they measuring?
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WA6BJH
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« Reply #13 on: February 22, 2011, 04:02:45 AM »

If I have a mobile HF antenna on my car, where are the losses?  The DC resistance loss is obvious; it's the DC resistance of the antenna rod and the coil.  This is not going to change much from antenna to antenna.  The biggest loss is ground resistance.  No matter how hard you work to bond doors and trunks and things like that together, the ground loss is what kills you on a mobile HF antenna.  The coupling between the car body and the ground--the dirt--is not good, in fact, it's terrible. 

If I unscrew my ATAS-120 from the antenna mount and put on some other antenna that's more highly rated, say 15 dB higher, what am I changing?  I'm changing the rod and the coil.  The one thing that I'm not changing is the ground loss.  It's still there and it's still exactly the same.  And the ground losses are the largest component of losses in an HF antenna.  If you had a way to move 120 radials around with your car, you'd have a pretty good antenna, and then changing antennas might make more of a difference. 

Where can the 15 dB difference come from?  It can't come from ground losses because we can't change the ground losses.  The ground losses on an HF mobile antenna swamp out all the other losses.  The only place the 15 dB difference can come from is the rod and the coil.   All the ground losses are so big that we're not picking up anything when we change antennas.  It's not quite the same as rearranging the deck chairs on the the Titanic, but it's similar.

So, back to the question.  What about a CB whip and an autotuner?  The ground losses are still going to be there, but here we would consider the loss at the input to the tuner.  I don't know what the loss in a tuner is, but I'm not surprised at K0BG's story.  The coils are small and the capacitors are small and I'm sure losses can be high.  The losses through a tuner depend on the settings, so the losses can vary a lot, depending on frequency. 

I have to go to work.

73
Kerry
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W5DXP
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« Reply #14 on: February 22, 2011, 07:17:03 AM »

How were the measurements taken?

The measuring setup was engineered by Bart Rowlett, WB6HQK, who if I remember correctly, was a airborne radar engineer for Hughes Aircraft. I didn't see the measuring setup, located a number of wavelengths away in the far field, but here's what I remember about the verbal description.

The transmit/receive measuring frequency was 3.95 MHz. The entry antennas were all positioned in the same location, one at a time. One of the entries was used as a test antenna at the beginning and end of the tests which lasted all morning. There was a 0.2 dB difference between the two test cases using the same setup, so scores were adjusted according to the time the test was run. It was on the CA coastline, was foggy to start, and cleared off close to noon.

At the source antenna, the forward power and reflected power were measured and the reflected power subtracted from the forward power to obtain the power delivered to the antenna. The various values were normalized to 100 watts, e.g. 3 dB was added to the score of an entry that delivered 50 watts to the antenna.

The receive antenna was a ferrite rod loop resonated by a capacitor at 3.95 MHz mounted on a pole up in the clear. A magnetic loop was chosen to minimize the effects of human bodies, etc. The measuring device was an expensive RF voltmeter. I don't remember the model number but I might be able to find out. The readings were in dB compared to some built-in reference.

Bart did this sort of stuff for a living and everything was consistent over a number of years. One of these shootouts was written up in Worldradio magazine but I can't find it.
--
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.
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