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Author Topic: What Screwdriver Antenna To Buy And Why  (Read 19380 times)
K0BG
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« Reply #30 on: August 16, 2010, 06:35:35 AM »

Here's what you have to do if you want a close estimate. From my web site:

Quote
In the Technical Correspondence section of the September 2006 issue of QST (page 57), are a few paragraphs written by Dr. Jack Belrose, VE2CV. Jack explains how to use an antenna analyzer and EZNEC to calculate the efficiency of a mobile antenna. The basic premise is to compare the measured input impedance of your mobile antenna, compare it to the modeled impedance given by EZNEC, and then adjusting the coil Q (resistive loss) until the two impedances (measured and calculated) equal. Then reading the programs calculated radiation efficiency.

The problem with this is, you don't know the ground losses, or radiation resistance with certainty. But perhaps, close enough as long as we admit some deviation percentage. Jumping through all of the hoops, the merit figure is always less than half the static Q. That is, at least when I've followed Jack's premise.

Assuming our guess about the Rg, and Rr is close to reality, we could measure the field strength. This has to be done rather exacting, and few (if any) amateurs have the equipment to do accurate field strength measurements. Again, assuming our error deviation, we could arrive at a closer figure for the coil's Q based on EZNEC's calculated field strength. I don't know, because I don't have the kind of equipment it takes to do accurate field strength measurements. This said, considering my hardware limitations, when I have tried the aforementioned scenario, the merit figure is always about half the static and/or advertised Q. Thusly, this is why I say it's about half, and in the cases of large end caps, about 60%.

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N5MOA
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« Reply #31 on: August 16, 2010, 08:56:43 AM »


Assuming our guess about the Rg, and Rr is close to reality, we could measure the field strength. This has to be done rather exacting, and few (if any) amateurs have the equipment to do accurate field strength measurements. Again, assuming our error deviation, we could arrive at a closer figure for the coil's Q based on EZNEC's calculated field strength. I don't know, because I don't have the kind of equipment it takes to do accurate field strength measurements. This said, considering my hardware limitations, when I have tried the aforementioned scenario, the merit figure is always about half the static and/or advertised Q. Thusly, this is why I say it's about half, and in the cases of large end caps, about 60%.





 However, as long as the Q remains above about 200 to 250, one is hard-pressed to measure the field strength difference.


Taking the Q number from HI-Q's site of 580 (I have a 5/160, that's the number I looked at) and a 60%(your number) reduction of Q  assembled, that leaves a Q of 232. That falls in your suggested range of "good".

But you also throw this opinion out:


 If a properly implemented cap hat is used (top of the whip), there is less reliance placed on the Q, as long as it hovers over ≈100. This isn't the case with the Hustler high power coils, and the HiQ series with their large metal end caps. In most cases, these coils operate very near self-resonance, and in some cases over it, which effects field strength in a very dramatic way.



So which is it?  A 60% reduction of the advertised Q because of metallic end caps, or a Q that doesn't hover around 100 because of metallic end caps?


73, Tom
N5MOA





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GOLDTR8
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« Reply #32 on: August 17, 2010, 05:46:07 PM »

Other than the debate on coil Q which I find interesting I have made my decision.

The most influence on my decision was from direct replies from folks who own different antennas and told me their experiences. 

My final choice is a Scorpion shorty. 

Although many strongly advised against a shorty, I have a height problem with my Jeep, and the shorty fully retracted met my criteria.  Second as discussed the end caps on the coil are not metal but a phenolic which helps with total losses as discussed by many in this post.  The final part was that I was sent real measured data of the scorpion antennas mounted in a vehicle where comparison data was done with different configurations and the shorty was only 1 dB down from the full size units.  Thus the choice was easy.

In the end I really appreciate everyone's feedback to guide me to a proper decision.  In the end the money was not as big an issue as I thought, as I can make choices on what is or is not important to me.

PS anyone interested in some used hamsticks :-) 
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N5MOA
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« Reply #33 on: August 18, 2010, 06:09:10 AM »

Congrats on the new antenna, and welcome to mobile hf, it's a lot of fun.

Although a "shorty" antenna from any manufacturer wouldn't have been my first choice, if it meets your needs, that's all that counts.

Now comes the fun (important) part, properly installing it. Alan does have some useful info on his site in regards to installs. I'm guessing you have already looked it over.

Other than the debate on coil Q which I find interesting I have made my decision.


I was enjoying the back and forth, but I haven't seen a clarification on my last question, so I guess the debate stopped.

73, Tom
N5MOA
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K0BG
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« Reply #34 on: August 18, 2010, 07:02:18 AM »

I don't know what went wrong, Tom, but I did post an answer to your question.

Anything you place within the field of the coil will reduce it's Q. This includes, but not limit to, end caps, lossy dielectrics, the body of the vehicle, the whip, the mast, and even the cover over the coil. How much reduction depends on too many factors to just take a stab at it.

It's been a few years since I tested a few Hustler loading coils with an HP-4342A, four wire Q meter, and I don't remember the exact readings. I do remember they were very close to those Jerry Sevick, W2FMI (sk), published in the original printing of this balun & unun booklet. If you take any of the Hustler super coils, measure their Q, then remove the end caps, and remeasure them, the Q just about doubles. I cannot believe that the end caps on the HiQ would have any less effect, especially when there is a shorting plunger within the coil's field as well.

As for the Scorpion 680S (shorty?), it's about as good as most full-sized HF mobile antennas.
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N5MOA
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« Reply #35 on: August 18, 2010, 09:26:44 AM »


Anything you place within the field of the coil will reduce it's Q. This includes, but not limit to, end caps, lossy dielectrics, the body of the vehicle, the whip, the mast, and even the cover over the coil. How much reduction depends on too many factors to just take a stab at it.


I'm not disagreeing with that.


It's been a few years since I tested a few Hustler loading coils with an HP-4342A, four wire Q meter, and I don't remember the exact readings. I do remember they were very close to those Jerry Sevick, W2FMI (sk), published in the original printing of this balun & unun booklet. If you take any of the Hustler super coils, measure their Q, then remove the end caps, and remeasure them, the Q just about doubles. I cannot believe that the end caps on the HiQ would have any less effect, especially when there is a shorting plunger within the coil's field as well.
 

I'm not disagreeing with this either. I'm sure it has some effect.

How much effect, and this



 If a properly implemented cap hat is used (top of the whip), there is less reliance placed on the Q, as long as it hovers over ≈100. This isn't the case with the Hustler high power coils, and the HiQ series with their large metal end caps. In most cases, these coils operate very near self-resonance, and in some cases over it, which effects field strength in a very dramatic way.



So which is it?  A 60% reduction of the advertised Q because of metallic end caps, or a Q that doesn't hover around 100 because of metallic end caps?


is what I'm disagreeing with.


It would seem it's hard to do, but if a coils Q can be measured static, why can't a coils Q be measured assembled, or an assembled antennas Q be measured?

Can't the same equipment it be used for all three tests?

If not, then all the numbers tossed around about assembled coil Q's are "I think" numbers based on calculations, conjecture, simulations and modeling. Nothing wrong with that, but all are as likely to be wrong as they are right.

How much reduction depends on too many factors to just take a stab at it.

That's my point.

All the manufactures that publish their static coil Q numbers should publish assembled antenna Q numbers. At least then we would know what we are starting with.  Then we can lower it more by actually mounting it on a vehicle.

Having said all that, properly installed, there probably isn't a ton of difference between most mobile antennas.

I have a HI-Q. I think it's the "best". Others think a different antenna is "best".

Who's on top is, obviously, a point of contention. That's why there are so many mobile antenna manufacturers. "Best" depends (IMO) upon your individual mobile needs/wants. Best for my needs/wants may not be best for the next guy.

Again, I'm not dissagreeing that anything within the field of the coil can reduce the Q. The amount of that reduction is the disagreement.

Until someone with the proper equipement (if there is such an animal) actually tests that reduction in the assambled antenna, we don't know.


73, Tom
N5MOA


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K0BG
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« Reply #36 on: August 18, 2010, 02:53:28 PM »

The only way to establish a reasonable figure for the installed Q, is to do a reliable field strength measurement. As I said before, very few amateurs (I said if any, but I know of a couple who could) have the hardware, and facilities to do an accurate field strength measurement. It requires some rather sophisticated hardware, and a very large field. What's more, you can't just measure at one TOA. Further, contrary to popular belief, you cannot rely on EZNEC or any other numerical engine to give you accurate data, as none of them do a decent job of calculating ground losses (among other things).

To my knowledge, HyGain was the last amateur radio antenna company to have their own test range. That range is now part of the University of Nebraska's sports complex in Lincoln, NB.

Until someone really spends the monies, and time, we'll have to rely on what little empirical data we have. Hopefully, that won't include any anecdotal crapola.
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WX7G
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« Reply #37 on: August 18, 2010, 03:20:38 PM »

With mobile antennas we are usually interested in RELATIVE gain measurements; antenna A vs antenna B; what is the DIFFERENCE in gain between A and B? Field strength measurements can tell us this and relative gain measurements are not difficult to perform. It is absolute gain measurements that can be difficult to perform accurately, but they are not needed to answer the question "antenna A or antenna B?"

So, field strength measurements are not required to determine relative field strength. A NEC model can tell us the radiation resistance VERY accurately. A base impedance measurement tells us the base impedance. Radiation resistance/base impedance and we have radiation efficiency. Using this method - if antenna A has higher radiation efficiency than antenna B - it has higher field strength and the field strength difference is equal to the ratio of efficiency.  

80 meter mobile antennas have such low radiation resistance that we often don't need to model it. The difference in radiation efficiency (and gain) is, for similar antenna electrical heights, proportional to base impedance.
« Last Edit: August 18, 2010, 03:23:41 PM by DAVE CUTHBERT » Logged
K0BG
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« Reply #38 on: August 20, 2010, 06:29:44 AM »

Dave, there is a problem with your model. You don't know with certainty what the radiation resistance is, and the one the program gives you is based on the calculated ground losses. Since these are is series with one another, when you change one, you change them all.

While I agree that field strength measurement can give you a comparison, you're no better off if you don't do the test correctly. Further, you can't do just one TOA, and then apply it as a better on, better two scenario.

I suggest you read what W8JI says about this very subject.
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KM6CQ
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« Reply #39 on: August 28, 2010, 12:10:45 PM »

Hi Dave,
I use the High Sierra Side Kick. http://www.hamcq.com/hf-antennas/sidekick-antenna/prod_3.html  This is my third screwdriver and the best I have owned. It is not as good on 75 as a full size one but it is worth the compromise. I can take it off the quick disconnect in seconds and throw it in the cab. I have been using it since 07 without trouble. I use it with the better rf antenna controller.  http://www.betterrf.com/7000-screwdriver.html  The rig is a IC-7000
This combo works with fault. I thought I could do a better job then the automatic controller since I have used screwdrivers for 25 years. But it is faster then me and positions the antenna just as well. If I was to do it again, it would be the same set up. If you use the antenna controller make sure you use RF beads.
Heres the set up.  http://picasaweb.google.com/bakerfreight/HFInstall?authkey=Gv1sRgCI7w7I2U3-6p1wE&feat=directlink

Good luck,    Dan
« Last Edit: August 28, 2010, 12:14:25 PM by Daniel L Baker » Logged
W6RMK
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« Reply #40 on: August 28, 2010, 06:06:12 PM »

I used to run a screwdriver (Nott ltd) and it worked fairly well.  However, don't get hung up on RF performance... they're all basically pretty much the same, or at least, other issues will dominate the performance.
1) How fast does it move to a new frequency?  How will you know when it's tuned there?  I put tape marks on the antenna that I could see, which got me close and then I'd just bump up and down til the SWR was lowest.  THere are some fancy controllers that count turns and remember for you, but....by the time you've bought that AND the antenna, there might be other solutions.
2) Where do you park? Do you need to be able to remove the antenna (or the whip on top)?
3) Where are you going to mount the thing.  About the worst would be bumper mounting on a van. I had mounted in two places on a VW Passat.. on top of the roof (a bracket on the luggage rack), and out in front of the front bumper.  The bumper mount was MUCH better and I could see it to see when band changing.

4) The rest of the system is critical.. how are you grounding it? where is the controller going to be mounted? etc.  My gut feel is that the ones made with converted screwdrivers (original design) aren't going to last as long as the ones made with an actual selected for the purpose gear motor.


--
I used the past tense above.. I got tired of waiting for band changes and fiddling with the switch to tune it while mobile.  I went to a SGC autotuner and dual resonance whip (a standard CB fiberglass whip with about 25-30 ft of wire spiraled around it and connected at the base.  MUCH easier to use, much easier to deal with in parking garages, and probably about the same RF performance (e.g. so much of the performance is just the physical radiator size.. the loss in the tuning network (SGC) or coil (screwdriver) is fairly small compared to everything else).  AND, about the same cost (I used the SGC that's not in a case, put it in a cable TV drop box, and the whip was essentially free)

http://home.earthlink.net/~w6rmk/mobile.htm
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KE5PPH
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« Reply #41 on: September 14, 2010, 07:58:31 PM »

Interesting set-up on the BMW. the body would be about the same for the OP's Jeep. I'm interested in trying to "mobile" my 04 LJ, tring to find the sweet spot for mounting the antenna. theres a fair amount of flat sheet metal around the rear of the vehicle, and a ball with a heavy spring will probably be the best mount. I think it would also alow the best place to get a good starting ground, along with bonding the tub. Do you think it would effect the tuner, if it was mounted inside the body? Using a 3-6 inch coax through the sheet metal to feed radiating element?
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W6RMK
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« Reply #42 on: October 08, 2010, 01:28:43 PM »

Coax of any length on the output of an SGC isn't a great idea (it puts a capacitor in parallel with the output, basically)..
Just use some flexible HV wire ( solid dielectric coax with the shield stripped off would do, so would test probe wire) and use some sort of plastic/rubber tubing or grommet to keep it away from the metal.  low capacitance is your goal here.
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KC4GS
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« Reply #43 on: October 11, 2010, 05:45:04 PM »

I have used a GS-3 10-160 on a big rig in 48 states and 9 Canadian areas, over a million miles.  It is a fixed length unit, and I have had very good results with it.  Over 10 years old, and the one time I thought I had broken something, it turned out to be only a paint crack on the fiberglass coil cover.  It is made by:     http://www.gs-mfg.com/pages/Products.html
I have had 2 of the moving coil types made by others before I got this one, one finally beat itself to death, the other I had all the way out, and hit an underpass.  Very loud nise and dead antenna.  Am getting stuff together to mount the GS-3 on a car now that I am retired, and try to get back mobile with my FT-900.

Mike
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