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Author Topic: Screwdriver vs. SGC 9 ft Whip/Tuner Combo  (Read 10423 times)
KG6TOP
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« Reply #15 on: October 13, 2010, 03:35:08 PM »

I have a firestick antenna on my truck for my cb, it gets the job done alright. I'm not trying to set any world records with it but it works fine for what I use it for.
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KI4SDY
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« Reply #16 on: October 13, 2010, 05:17:14 PM »

The stainless steel whip and the auto tuner will be easier to maintain and operate than the screwdriver antenna for sure, especially in your situation. The screwdriver antenna can only be tuned safely while stationary when driving a large commercial vehicle. You need to keep your eyes and mind on the road as much as possible, especially if I am driving next to youWink

The screwdriver antenna will probably perform better, until you hit that low limb with it mounted on the mirror. Then you can go pick up the parts!  Grin
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K0BG
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« Reply #17 on: October 14, 2010, 07:02:02 AM »

Most of the conclusions reached in this thread are based on lore, not fact! Any antenna will work, but one has to define the term work.

Almost without exception, anything that Firestix makes is based on anecdotal nonsense that seemingly dominates the landscape. Since their whole marketing effort is to sell antennas, they aim their ad campaign towards the least common denominator. As Lon said, is it so much hype.
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KG6TOP
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« Reply #18 on: October 14, 2010, 12:21:05 PM »

The only reason I got that firestick antenna is because it was cheap
. I really dont care about "getting out" or whatever it is that those cb whacker shootout guys call it. The only thing I use my cb for is communicating with customers and dock workers. 95% of the time its turned off since I just don't want to listen to all the garbage. In closing, my fire stick antenna does exactly what I needed it for, short distance communications. Plus it was cheap.
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W6RMK
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« Reply #19 on: October 15, 2010, 07:27:16 AM »

Screwdriver is nice if you don't change frequencies much, but it does take time to get it dialed in, particularly on 40 and 75.  The autotuner is instant.

Radiation effectiveness wise, they're probably about the same.  The overall physical size of the radiator is similar, so all the stuff about where the loading coil is, how big it is, etc, just changes how much loss there is (or in which components it's dissipated)  in matching from the 50 ohms (or so) that the Tx wants to see and whatever the antenna is.  As other posters have noted, performance depends a lot on things like where the grounds are, etc.

Cost wise.. screwdriver is cheaper, a bit, but if you get the bare SGC and put it in your own box, it's pretty competitive at the low power end of the scale.  If you want to run a kilowatt, the autotuner gets pretty pricey, compared to a screwdriver with bigger wires.

The SGC whip *is* different than a standard CB whip.  It has two wires in it, so it has a resonance down lower (around 9 MHz or so, but maybe lower... I can't remember).  Doesn't change the radiation effectiveness (physical size is the same), but it DOES help make it easier to match.  You can do the same by taking 25-30 feet of AWG12 house wire and spiraling it up your CB whip. 
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W5DXP
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« Reply #20 on: October 15, 2010, 06:10:05 PM »

Screwdriver is nice if you don't change frequencies much, but it does take time to get it dialed in, particularly on 40 and 75.  The autotuner is instant. Radiation effectiveness wise, they're probably about the same.

Nope, they are not - especially on the lower bands like 75m. The whip+autotuner has been measured as ~14dB down from the top rated screwdrivers. That's close to 3 S-units! The radiation resistance of a good 75m screwdriver is about one ohm. The radiation resistance of a 9' whip on 75m is considerably less than one ohm and autotuner losses have to be taken into account.
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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.
W6RMK
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« Reply #21 on: October 15, 2010, 09:21:37 PM »

I'd like to see those measurements and the configurations tested.  A factor of more than 20 is pretty big.   
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K0BG
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« Reply #22 on: October 16, 2010, 06:48:04 AM »

Jim, the difference Cecil speaks of is close to real-world. In some cases, however, the losses are even worse!

Any antenna tuner, coupler, matchbox, whatever you wish to call it, will have problems matching impedances lower than line Z. That is to say, throughput losses get really big in a hurry. It gets even worse as the Z drops below about 20 ohms, when a major portion of the loss is in the network, no matter the network's configuration. The SGC is a bit better than a standard LC, as in some situations, it sets itself up as a Pi network.

Then there is the stainless steel whip itself. Almost all whips are made from 17-7 stainless steel. The losses through it are much to write home about, until the frequency drops below 10 MHz. At 3.5 MHz, the resistive losses through a 102 inch whip (standard CB) accounts for about 20% of the overall system loss, no matter whether an ATU or loading coil is used.

Further, the ATU/whip combination is essentially a base loaded antenna system, which means the radiation resistance will be about half that of a center loaded antenna, all else being equal. Since all of the losses (except stray losses) are in series, changing one, affects them all. So, the only real way to compare any given two setups, is to measure field strength. That's a problem too, as most amateurs do not have the correct equipment to make field strength measurements accurately, antenna shootouts to the contrary.

Owen Duffy, VK1OD, has done some very recent research on the aforementioned scenarios.

http://vk1od.net/antenna/conductors/loss.htm

http://vk1od.net/antenna/multibandunloadedvertical/13mEV.htm

Both of these articles make interesting reading.

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W6RMK
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« Reply #23 on: October 16, 2010, 08:18:41 AM »

Adding up all the effects you cite doesn't get you to a factor of 20, though.

The loss in the tuner is going to be comparable to that in the loading coil of the screwdriver, eh... Same diameter wire, about the same current, etc

As for the whip, yes ss has higher resistivity, but if you're doing the sgc whip or home brew equivalent like I did, it's copper.  And in any case, the 20% you cite is only 1-2 dB

Radiation resistance might be slightly higher, but really consider the best you can do for a given size physical radiator. Uniform current over the entire length vs the cos() current profile. That's maybe a difference of a factor of 2.

I can see a difference of 4-5dB, for best case screwdriver vs worst case tuner/whip.  Not 14dB

And I think that in real installations there may be only a few dB difference either way, and other installation aspects will dominate.

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K0BG
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« Reply #24 on: October 16, 2010, 09:08:13 AM »

Quote
Adding up all the effects you cite doesn't get you to a factor of 20, though.

Unfortunately, it does.

Quote
The loss in the tuner is going to be comparable to that in the loading coil of the screwdriver, eh... Same diameter wire, about the same current, etc

Unfortunately, that's wrong too. You're trying to compare loaded Q with unloaded Q, and you can't do that.

Quote
but if you're doing the sgc whip or home brew equivalent like I did, it's copper

The SGC whip isn't just a straight piece of copper, as it does indeed have a loading coil. One model of it has two loading coils. The Q is rather low, as it hovers around 50. Remember, these coils are small in diameter, and wound with what appears to be #20. Even if your setup is a solid piece of copper, you still have the ATU losses to contend with.

You can get the ATU loss numbers by using a simulator like this one: http://fermi.la.asu.edu/w9cf/tuner/tuner.html Running the numbers is easily done using EZNEC. You can use the ground loss figures from the ARRL Handbook, or Antenna Book. Ground losses are typically larger than those given, but for this exercise, they're adequate.

When you run the numbers you'll find that the difference in some cases, particularly on 80 meters, is more like 30 dB. It's less on 20 meters obviously, but it's still about 12 dB to 15 dB depending on the overall length, and what the element is made out of.

All this assumes you're feeding the antenna element directly out of the ATU. If you're using coax between the whip, and the ATU, you'e really going to be surprised!
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W5DXP
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« Reply #25 on: October 16, 2010, 09:59:59 AM »

I'd like to see those measurements and the configurations tested.  A factor of more than 20 is pretty big.

At the bottom of this page is a summary of three 75m CA shootouts. Both the top rated antenna and the SGC autotuner with 11.5' whip were mine, installed on the same GMC pickup, so the -12 dB (factor of ~15) result leaves no doubt. It would be even worse with a 9 foot CB whip, 2.5' shorter than the 11.5' whip used for the text. I estimated the difference between a 11.5' whip and a 9' whip on 75m to be 2dB using EZNEC to estimate the feedpoint impedance of each whip.

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

I can see a difference of 4-5dB, for best case screwdriver vs worst case tuner/whip.  Not 14dB

Actually, the measured difference between the tuner/whip combo and a base-loaded bugcatcher system is 7dB. And the base-loaded antenna is 5 dB down from the center-loaded screwdriver system. 7dB+5dB = 12dB.

Quote
The loss in the tuner is going to be comparable to that in the loading coil of the screwdriver, ...

One thing that you seem to be missing is that a considerable amount of radiation comes from the high current bottom section under the coil which doesn't exist in the autotuner/whip combo. In fact, simply moving the loading coil from the center position to the base position costs 5 dB of available radiation according to actual measurements. Where is that 5 dB in your estimates?

Point is that a lot of radiation takes place before the RF reaches the center-loading coil. That radiated RF cannot be dissipated in the center-loading coil but it can certainly be dissipated in the autotuner.
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73, Cecil, w5dxp.com
« Last Edit: October 16, 2010, 10:53:37 AM by Cecil A. Moore » 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.
W6RMK
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« Reply #26 on: October 16, 2010, 06:20:55 PM »

Interesting data, Cecil, but you don't give any measurement methodology..

First, what we're comparing here is screwdriver vs sgc & whip (so we don't have to worry about bugcatchers/capacity hats/etc)

I think the antenna tuner losses are smaller than everyone is claiming.
I haven't measured the loss, but here is the measured feedpoint impedance for my improvised dual resonance whip: low resonance around 6.6 MHz, but I didn't specifically plan it that way... 25 ft more or less of AWG14 , spiraled up a 10-11m band mobile antenna (bottom half fiberglass rod with copper wire, top half SS whip).

I then took the Z and ran it through W9CF's tuner simulator, which simulates a CLC T network, as opposed to the Pi network in the SGC. But I would think, off hand, that the losses are comparable.
MHz  Z              Loss
3.23 78.3-209i  11.2% 0.5
3.48 64.5-174i  11.3% 0.5dB
3.73 64.2-146i  10.1% 0.5dB
3.98 60.6-126i   9.6% 0.4dB

6.95 137 -325i   8.1% 0.4dB
7.20 104 -267i   8.2% 0.4dB
7.45 82.6-224i   8.4% 0.4dB

So it looks, at first glance, like the loss in the tuner is going to be less than a dB.
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AD5X
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« Reply #27 on: October 17, 2010, 04:52:42 AM »

If your impedance measurements are correct, you have very high ground losses.  As stated earlier, the radiation resistance of a 9-foot antenna on 80 meters is less than one ohm.  Yet you are showing a very high real impedance there.  Since your antenna resonates at 6.6 MHz, check it there and see what the impedance is.  If you had a 1/4-wave vertical over perfect ground you'd see 37 ohms.  For a 9-foot resonant antenna over perfect ground, I'd expect 2-3 ohms of radiation resistance.

The ARRL has run some loss tests on autotuners, and losses of 20% or so on 80 meters are not unusual (about 1 dB) when matching lower impedance loads.  However, these losses are measured into real resistances.  When you match into the high capacitive reactance of short antennas, you increase the amount of inductance necessary, so losses due to inductor Q will increase.  Inductors in SGC and other autotuners are typically close-wound, which generally means less than optimum Q.

Here's something to try.  Disconnect the output of your SGC tuner and then see if it will tune on 80 meters.  I bet it will.  So what is it tuning into?  It has to be the internal tuner losses.

Phil - AD5X
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K0BG
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« Reply #28 on: October 17, 2010, 06:38:55 AM »

If you do test the SGC as Phil suggests, use the least power you can to get it to tune, lease you damage it.

The R value Phil gave you for a 9 foot whip is very close to reality. The Rr would be ≈.4 Ω plus whatever resistive value the element has at the given frequency. If that element was 17-7 stainless steel, you'd have to add about 2 Ω, which ends up being very close to Phil's figure. Any matching network, no matter what is is, will have problems matching resistive loads this low in value. Even if they could find a match, the throughput losses would be excessive (>85%), resulting in a destroyed ATU. In the May 2004 issue of QST, the ARRL tested the SGC ATU in question. Up can pull up that article on-line, on the ARRL site.

As I stated before, Owen Duffy, VK1OD, has done some further testing of the ATU/whip combination. He calculations echo what's already been said here. While all of this is a rude awakening perhaps, it is technically sound.
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W6RMK
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« Reply #29 on: October 17, 2010, 09:56:39 AM »

These are all good comments.
And yes, the R component is remarkably high, so when it stops raining, I'll have to redo the measurements (which were done a few years ago)..

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