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eHam Forums => Mobile Ham => Topic started by: KK7HO on September 21, 2009, 06:00:58 PM



Title: Antenna shootout results
Post by: KK7HO on September 21, 2009, 06:00:58 PM
Here's a link for those that may not have seen the antenna shootout results. The Scorpion Antenna made a great showing. I've had one in operation for over a year and it hasn't missed a beat.

http://www.scorpionantennas.com/3.905-century-club-shoot-out-results-2009.html


Title: Antenna shootout results
Post by: K0BG on September 22, 2009, 09:43:04 AM
You can bet that one six-lander is other than pleased!

Alan, KØBG
www.k0bg.com


Title: Antenna shootout results
Post by: K6TFZ on September 22, 2009, 04:15:09 PM
It's expected that the Scorpion did so well because the coil was so high and in the middle of the pickup bed with lots of metal bonded and under it. What I'm interested in is why the Hi-Q did so poorly compared to other antennas which were mounted similarly to the Hi-Q.  Geoff, K6TFZ


Title: Antenna shootout results
Post by: K0BG on September 22, 2009, 05:55:17 PM
It's simple actually. The cap hat used was mounted too close to the coil. If you want additional information about this, go to my web site and read the Cap Hats article.

There is another reason too, which becomes even more apparent as you increase the frequency of operation. Self-resonance as it were.

The end cap of the HiQ coil is aluminum, although stainless steel is available. When you move the plunger towards the top, both it and the end cap greatly effect the Q of the coil. As a result, it is possible to use a large cap hat, and tune the antenna to resonance on 17 meters, yet use the antenna on 15 through 6 meters with less than a 3:1 SWR. The occurs because the metal within the field of the coil, causes it to look more like a capacitor instead of an inductor. Or, as I said, operate above self resonance.

If the cap hat is large enough, like the reentrant one used, and mounted too close to the coil, the losses in the coil become very large, no matter the static Q.

The bottom line is, you lose more than just a questionable contest.

Alan, KØBG
www.k0bg.com


Title: Antenna shootout results
Post by: KK7HO on October 26, 2009, 10:09:26 PM
I'm sure that 6-lander is displeased - poor sport & sore loser.


Title: Antenna shootout results
Post by: KH6AQ on October 27, 2009, 09:55:57 AM
Given the variables of the vehicle, the shootout results are ok for owner bragging rights but are meaningless for comparing antennas.

To fairly compare antennas they each need to be installed on the same platform.

The pickup truck with the bed antenna mounting provides a clean test platform.  

Manufacturers should not use data biased in any way for advertising purposes. This particular shootout data is biased by the vehicle installations.


Title: Antenna shootout results
Post by: WB5JEO on October 27, 2009, 12:13:18 PM
Well, it's not really an "antenna shootout," is it? It's various antennas mounted in various ways on various vehicles with likely some variations in grounding, feeds, etc. Looking back over all the years' results, it's pretty clear that any number of different antennas can come out on top in any given year, some of them clearly not on account of the virtues of the antenna alone. In other words, a casual bragging rights sort of fun thing.

But I was amused by the lawyer letter on the site. Four out of five dentists agree that's dumb. Follow the practice enforced (deadly serious about it) on their field reps in the 70's by IBM. Never say anything bad about the competition, even when they speak ill of you. And don't make threats, especially silly ones. Quoting Tuco (The Ugly): "When you have to shoot, shoot, don't talk."


Title: Antenna shootout results
Post by: KJ4DKT on November 21, 2009, 05:36:13 PM
today i mounted a h/s sidekick on the roof of my truck w/a mag mount (T_-bar).  the manual says to not do it this way.  i tuned it to 20m and talked to Bahamas, S. Dakota, Michigan, and other states always w/ s9 and better.  the magnets are bare and the paint on the roof is bad.  this may be the reason.  the mags are directly grounding to the sheet metal.  i have no grnd straps.. IT WORKS BEAUTIFULLY!!!!!   im leaving it alone and continue to talk my head off.  your milage may vary.   fred   kj4dkt  it also tuned up to 10m w/no probs..i will try 75m tomorrow.


Title: Antenna shootout results
Post by: K5END on November 23, 2009, 11:37:27 AM
One question (among others) I have about the shootouts is the method of measurement.

If I understand correctly, all the remote measurements are taken at ground level "several wavelengths away." If that is correct, then the data isn't worth a lot (which everyone seems to agree upon anyway.)

How about measuring at 10 degrees elevation from 20 or more wavelengths away? It wouldn't be easy to do this, but it could be done.

For 80 meters that would require a tower at slightly more than one mile distance and about 300' high. Surely it's not hard to find flat, unobstructed land within a mile of a 300' tower or hill, right?

Another way is to use a nylon-guyed helium blimp (commonly available) and attach the calibrated antenna to it. That method also offers an easy way to measure signal strength as a function of height, continuously. All you need is a winch or two (but a few wenches will work just as well.) I'm certain that Hams are innovative enough to do this.

Instead of having a circle of guys around the azimuth making measurements around the vehicle, just rotate the vehicle or test platform. It could be done in small increments this way and probably be more consistent.

Above all, a calibrated transmitter and antenna should be used throughout all the measurements during the day to normalize the data against any environmental changes.

If we're going to get po'ed about a shootout, at least let us get scientific about the method so we can know exactly what we are getting po'ed about.

Just by looking at the data from this year's shootout, I'd say the method needs some improvement. There are obviously hidden variables. Some of it makes no sense at all.


Title: RE: Antenna shootout results
Post by: K0BG on November 28, 2009, 05:26:29 PM
Larry, you've brought up some very valid points, and ones I've tried to cover in detail in my Antenna Shootouts article on my web page. However, there is a little more to it.

I agree the measuring methods is suspect, the elevation issue is valid, and so as the others you put forth. But, the intension was to be as far and unbiased as possible, So, let's assume they were as I say, unbiased (at least one manufacturer doesn't believe it was unbiased). The measurement method consists of a Faraday loop, fed to a full-bridge diode array, with a few intermittent pieces, and fed to digital voltmeter. The only thing that does is compress the higher signal levels together (logarithmic scale like as SWR bridge). That fact might be a problem if the top few best score are very close together, as their difference is within the accuracy fuzz. It would be nice if the measurement was done with a calibrated receive antenna, but those things aren't cheap.

The transmit relied on a Bird and a transceiver to assure the same power was used. Probably a better solution, if you were reliant on an amateur transceiver, would be to use a large attenuator in the transmit line to assure the meter, and the transceiver weren't all that effected by a change in antenna input impedance. Even adjusting the match on each antenna with an analyzer isn't all that accurate in this case.

Aside from all of this, everyone in the shootout was under the same rules, level of bias (we assume none), so whatever the outcome, even though it is not linear (a given), is the same for all. If you enter, you accept the method. If afterwards, because you lost, you B, M, & C, threaten law suits, and make demands through your lawyer, opens up a whole new set of dynamics.

Digressing. The TOA angle 'thing' is very important, and points out one verifiable flaw. You can present a case, wherein the winner was actually the loser with respect to DX. The average incoming angle for multi-hop is about 15°. Since ground losses effect the low angle signal levels, without measuring at different angles, you have no way of knowing who is best.

This said, in this case the measurement angle was very shallow, which in effect negated what I just said. It takes some thought, but I believe you see the picture as it is.

Alan, KØBG
www.k0bg.com


Title: RE: Antenna shootout results
Post by: AA4PB on November 28, 2009, 06:57:42 PM
I also submit that useful data would be provided if shootouts were run on other bands rather than just 75M. I understand that 75M represents a worst case (other than 160M) but many mobiles use bands other than 75M. While a good screwdriver will severely trounce a Hamstick on 75M I expect that the difference would not be nearly so severe on 20M or 15M. Data on those bands would permit people who are interested primarily in the "DX" bands to make a better judgment concerning trade-offs for those bands. Perhaps something in between the two.


Title: RE: Antenna shootout results
Post by: N5XTR on November 28, 2009, 08:00:17 PM
I remember a QST article comparing different types of mobile antennas.  The screwdrivers didn't fare very well.  November 2007 QST page 30.


Title: RE: Antenna shootout results
Post by: K5END on November 28, 2009, 08:24:07 PM
Alan,

It appears a better method may be beneficial for the contest. I don't know who does the existing "shootout," and I mean no disrespect to them.

It's not even any of my business to tell them what to do or how to do it. It's their party. No one is required to participate, or even take the results seriously for that matter. I do kind of see the point of the disgruntled participant, as I infer his complaint to mean that some people might take the contest seriously. I think a shootout disclaimer is needed, again, in my opinion.

What is confusing to me is that your web page sings the praises of some of the very same design criteria used in that design. And your web page casts a doubtful eye toward the validity of antenna shootouts. Or at least it used to. I have not seen your page for a while.


Title: RE: Antenna shootout results
Post by: K0BG on November 29, 2009, 06:40:13 AM
I more than mention what follows, but it isn't evident on the outset Larry. And, I'm looking at the verbiage so the subject is better explained.

Assuming the Q of the loading coil exceeds 200 to 250, any additional increase has very little impact on the actual field strength. However, if it is less than say 150, you might even notice it aurally if you have a long-term mobile experience (no offense meant). Drop the Q suddenly to say 50, the noise level comes up (S+N/N), and contact start getting difficult especially under noisy band conditions.

If we assume we're comparing two antennas, a screwdriver and a plunger, their Q curves as you adjust their frequencies, becomes a study in loading coils.

In a screwdriver, the Q on the low end is about the average for the antenna, and it increases as you move up. Then it falls some on 20, but starts going back up again. Remembering the coil is not shorted on the bottom end, but you still have circulating currents in the unused portion of the coil. There is capacitive coupling between coil and the inside of the mast which affects the current, and thus the Q. Remember, as you go up in frequency, more of the coil is swallowed by the mast, and the coupling between the coil and the mast increases too.

In the case of the plunger, there are two factors at play. The bottom of coil is electrically connected to the plunger. This shorts out the turns, and lowers the inductance of the coil. These shorted turns also reduce the Q. The effect is small until you get about half way. The plunger is metal, and so is the top end of the coil (end cap). When the plunger moves close enough to the end cap the coil Q drops precipitously, and the coil's self resonant point is reached. At what point this occurs depends on the coil's LD ratio, initial Q, etc. If you're using a long whip and/or cap hat, it might occur as low as 14 MHz. To be honest, I didn't realize how bad it was, until I borrowed the equipment to measure it. Sort of a rude awakening. Aside from that, properly made, a plunger is somewhat stronger than most screwdrivers; a fact I've tested several times!

As Bob pointed out, the difference in signal strength between a Hamstick and a decent screwdriver isn't much on the upper bands, especially 17 and above. But it's still close to an S unit, which most folks can live with. This said, if a shootout were done on 20 meters, any antenna which used a shorting device, even one without the metal end caps, would be at a great disadvantage, and could be easily defeated by a Hamstick.

I'll leave you with this thought. In the shootout in question, the plunger used two cap hats, both close to the top of the coil than the end of the whip. The coil, and both cap hats were within 14 inches or so of the body of the vehicle. All of these facts, are coil Q robbing factors. That's why free and clear makes more sense most of the time, than selecting an antenna with better Q coil.

Alan, KØBG
www.k0bg.com


Title: RE: Antenna shootout results
Post by: K5END on November 29, 2009, 12:21:36 PM
If we assume we're comparing two antennas, a screwdriver and a plunger...

Are you saying that the unused portion of an unterminated section ("end") of coil, with the windings in the unused section not shorted together, inside the mast couples with the mast itself and creates an unwanted LC circuit as part of the antenna? Have you modeled this to see what happens and if that stray circuit is significant?

Can you give an example of a plunger and a screwdriver? I'm guessing you mean a plunger is an antenna that uses finger stock at the top of the mast tube to eliminate part of the coil, and the motor is contained near the bottom of that tube. How is that not a screwdriver antenna?



Title: RE: Antenna shootout results
Post by: K0BG on November 29, 2009, 01:10:28 PM
As for plungers. One made in California, one that looks more like a screwdriver made down in S. Carolina, and another made in Ohio, use a fixed coil. The bottom of the coil is electrically connected to a plunger which moves up and down inside the coil, although the one from Ohio is on the outside of the coil. As the plunger moves up, more and more of the coil is short tapped. Short tapping isn't ideal as doing so reduces Q. Whether this is a detriment, depends on a lot of factors. In one case, the top end of the coil is a large chunk of aluminum. Even when the plunger is all the way up, some coil is still active, and caught between the metal plunger and the metal end cap. Under the right circumstances, the coil operates above self resonance, and the losses are almost massive.

In the traditional screwdriver, the coil is swallowed by the mast. At the top of the mast are contacts which circle the coil. The top of the coil is connected to the whip, obviously. The lower end of the coil is inside the mast. It doesn't actually contact the mast, but in some models it does.

Cheaply made screwdrivers often have enough slop that the bottom of the coil that it is possible that it contacts the mast, and when it does the coil's Q and resonant frequency changes. I know of two that do this, and it is very evident when you're traveling down the highway. At first, I didn't know what was causing the problem, but did manage to figure it out in due course.

I've put a lot of thought into this, and I can't see an easy way to measure the current in the unused portion of the coil. About the only thing you can 'see', is when the gap between the coil and the mast gets too large. There can be arcing between the lower sections of the coil, and the interior of the mast. I personally have seen the results of this happening when playing with inexpensive screwdrivers. Based on nothing other than that, one has to draw a conclusion that circulating currents can be significant.

As Belrose points out in many of his articles, the best solution is to have a coil of the exact size needed with no shorted turns. I'll buy that. But the rest of us like the idea of changing bands while under way. The least wind loading is offered by one of the two types mentioned. However, when compared to a fixed coil, both methods are a compromise Q wise, all else being equal. Since so many factors are at play, it's difficult to calculate and/or measure reductions in Q. About the only thing you can do fairly reliably, is measure field strength, and compare that to the same antenna modeled with EZNEC or NEC4. Belrose wrote about this methodology in an article which appeared in the ARRL Compendium #4. I think I've read that article 10 times.

I am not encumbered with massive amounts of test gear, and I don't have the real estate either. This sort of leaves me on the short end, so to speak. In any case, if one really had the time, inclination, and equipment, testing a few models of each over their operating range, would certainly be enlightening.

Alan, KØBG
www.k0bg.com



Title: RE: Antenna shootout results
Post by: KH6AQ on October 05, 2012, 07:50:08 PM
I ran across a reference to this antenna shootout and the Hi-Q company again today. Looking at the test conditions it is VERY obvious why the Hi-Q antenna did so poorly. It was mounted on the rear bumper of a van! Mounted on the bed of a truck as the "winning" Scorpion antenna was the Hi-Q might have been the winner.


Title: RE: Antenna shootout results
Post by: W8JI on October 08, 2012, 08:00:47 AM
You'd be surprised at how little loss of Q often occurs from shorting turns under the right application.

On the other hand, there can be a huge problem from not having shorts at multiple places in unused coil.

Look at a Smitch chart of a roller inductor near the bottom of this article ("The Roller Alone" section):

http://www.w8ji.com/tuner_baluns.htm

Loading coils for antennas can have the same problem.

73 Tom
     


Title: RE: Antenna shootout results
Post by: W8JI on October 08, 2012, 08:48:29 AM
One question (among others) I have about the shootouts is the method of measurement.

If I understand correctly, all the remote measurements are taken at ground level "several wavelengths away." If that is correct, then the data isn't worth a lot (which everyone seems to agree upon anyway.)

How about measuring at 10 degrees elevation from 20 or more wavelengths away? It wouldn't be easy to do this, but it could be done.


I might be wrong, but I'm guessing you (like "Woody") do not understand antenna patterns in commonly used Ham programs.  The patterns we see in EZNEC  are based on calculations at a very long distance from the antenna over a flat ground media. The calculation distance is so large that any low angle signals are forced to near zero by earth attenuation.

At casual glance, without thinking about how pattern is calculated, this gives us the false impression peak radiation is at some angle higher than zero degrees. While the model is accurate at distances of hundreds of wavelengths over a flat homogeneous earth, it doesn't represent the elevation pattern at reasonable distances at all.

Misunderstanding  pattern calculation methods causes us to accept TOA as being accurate for verticals, and come up with all sorts of weird theories about vehicle chassis radiation. It makes us *think* we should measure at some higher angle to be in the main lobe. This would indeed be true if we were measuring ten or twenty miles from the antenna, but the simple solution is to not measure twenty miles away where groundwave has deteriorated to zero.

Let's be clear about this. TOA and low-angle elevation pattern does NOT apply to vertical antennas unless we are at infinite distance. We can certainly use model pattern to compare efficiency between verticals of similar height, or to see if some ground systems are radiating at high angles, but it is a terrible mistake to think the pattern at some huge distance over flat earth in away way represents things at modest distances.

There is nothing wrong with comparing short vertical elements on groundwave at distances of a wavelength or two.

Now if an antenna is mounted out on a vehicle's end there certainly can be pattern distortion, and there might be a little radiation from the chassis, but radiation is minimal. The earth tends to cancel any high angle chassis radiation, and the chassis is normally fairly wide and tall compared to length.

I don't see anything wrong with a 1-2 wavelength distance groundwave measurement of short vertical antennas.
 
73 Tom


Title: RE: Antenna shootout results
Post by: W5DXP on October 08, 2012, 03:08:43 PM
While the model is accurate at distances of hundreds of wavelengths over a flat homogeneous earth, ...

I wonder how many hams are members of the flat earth society? :)