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Author Topic: Real mobile ant measurements  (Read 5629 times)
WX7G
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« on: July 24, 2010, 01:03:24 PM »

Four 40 meter mobile antennas were measured for base impedance when ground mounted over sixty four 20' radials. The ground loss is estimated to be 5 ohms.

  • Short MFJ, 53", 24 ohms base, 1 ohm calc radiation resistance, calc coil Q = 150, 4% radiation efficiency
    Hamstick, 95", 28 ohms base, 3 ohm calc radiation resistance, calc coil Q = 120, 11% radiation efficiency
    Hustler RM40, 92", 25 ohms base, 3 ohm calc radiation resistance, calc coil Q = 120, 12% radiation efficiency
    Tarheel 200A, 106", 16 ohms base, 3.7 ohm calc radiation resistance, calc coil Q = 240, 23% radiation efficiency

The Hamstick is 4 dB over the short MFJ. The Hustler and the Hamstick are the same, and the Tarheel is 3 dB over the Hustler.

From what is considered a real compromise antenna to what is considered a good mobile antenna the difference is nearly 8 dB.

I'll perform field strength measurements to compare to the base impedance derived relative gain measurements and post them when they are completed.
« Last Edit: July 25, 2010, 10:50:36 AM by DAVE CUTHBERT » Logged
W5DXP
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« Reply #1 on: July 24, 2010, 02:32:42 PM »

Four 40 meter mobile antennas were measured for base impedance

Good stuff, David. The 7dB spread between a hamstick and a screwdriver on 40m goes up to ~10dB on 80m and down to hardly noticeable on 10m.

Minimizing the number of electrical degrees occupied by the loading coil seems to be the key to radiation efficiency for mobile antennas.
--
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.
WX7G
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« Reply #2 on: July 24, 2010, 05:10:26 PM »

Cecil, yes the efficiency numbers should be very roughly 1/4 on 80 meters and 4X on 20 meters.

 

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W4FID
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« Reply #3 on: July 27, 2010, 03:12:01 PM »

Longer is better ........... seems easy enough to believe even to us not too technical guys. Knowing actual dats is nice too. Thanks for both the effort and for publshing it.

However; I think there is another factor in play for mobile. I have an F-150 pick up and use a BIG mag mount in the center of the roof. I use the "mini" hamstick type which are bout 2/3 the length of the "normal" full size ones. That puts the tip about 11'-6 off the ground. With a small spring I do fine. It does tonk some tree limbs and even an occasional overhang like at the bank drive thru. But being careful and slow when needed and the spring allows it to be a viable installation. A full size stick would be about 15 feet to the tip and just too high. It would require a larger spring and be hitting everything including some underpasses at highway speeds. Not a viable installation.

I feel there is a compromise between length and mounted location. In the center of the roof it is symetrical -- better coupling to the vehicle and less directivity Vs a corner fender mount. Also it's above my sheet metal; and the car next to me's sheet metal; and guard rails; etc. If it was frame or bumper or fender mounted with the base maybe a foot to two and a half feet high it would be low enough to clear obstructions -- but suffer loss the center of the roof mount doesn't have. The lower portion -- the part that has the highest current and radiates the most -- puts the RF into the sheet metal less than a foot away. I've used both for several years each and feel over all the performance is about the same based on on the air results. Who I can work on the county hunters; how easy it is to check into SOCARS or MIDCARS; how solid and easy QSOs are as I drive.

But with the shorter ones on a qucik disconnect I can easily stick them in the truck. The long ones are a pain to store.

I do keep a couple full size ones in my travel trailer and use them when stationary and QRP from an RV park or campgrouns as they are better when hitting overhead stuff isn't an issue.

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WX7G
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« Reply #4 on: July 27, 2010, 04:24:28 PM »

Yes higher is better. The vehicle is a radiating portion of the antenna; the vertical length of the vehicle contributes to radiation. You have an 11' antenna.

I'm trying out various antennas on my car. The antenna is mounted at 5' above ground level. An angled hamstick works well with the top 10' above the ground. I'm presently using only the coil and stinger of a Hustler RM40 resonator angled and that makes for a 7' long 'antenna' counting the vehicle. I have not yet modeled this but a paper calculation says it should be down 3 dB from the hamstick. With 400 watts it does the job on 40 meters.

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K0BG
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« Reply #5 on: July 27, 2010, 05:57:02 PM »

I'm sorry Dave, but I don't buy any of it.

An RM40 with, or even without, the end caps, measured on the bench (not part of the antenna), has a measured Q of under 60. And, there is no way a 40 meter Hamstick has a Q of 120. The only one that is close is the 200A tarheel. It's static Q on the bench, is close to that stated. Once it's installed as part of the antenna, it is considerably less.

Where on earth did you come up with this Q ratings?
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WX7G
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« Reply #6 on: July 27, 2010, 08:22:01 PM »

Alan, as listed in my post these are calculated Q values. They are based on base impedance measurements. Note that this is the RM40 and not the RM40S resonator.  

I'll measure these two inductors on a lab instrument and post the results here. This will allow me to close the loop and see if the indirectly-measured and calculated coil Q numbers are good.

Based on antenna base impedance measurements I calculate the Hustler Q = 120 and the Hamstick Q = 120. Using a coil calculating program the Hustler Q = 170 without end caps and the Hamstick Q = 100. Measured and simulated are the same ballpark.

Let's see what a Q of 60 does to the Hustler antenna. The RM40 is 48 uH. At 7 MHz, neglecting interwinding capacitance, the reactance is +j2100 ohms. If the coil Q was 60 the coil resistance would be 35 ohms. The input impedance of the antenna over my 5-7 ohm ground would be 41-43 ohms. But I measured 25 ohms. So, it does not look like the Q could be as low as 60.
« Last Edit: July 28, 2010, 04:55:03 AM by DAVE CUTHBERT » Logged
K0BG
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« Reply #7 on: July 28, 2010, 09:33:14 AM »

The problem is Dave, you cannot rely on base impedance measurements. The Q losses, radiation losses, ground losses, and conductor losses are in series with one another. Guess one, and you're guessing the all!

The Q of the small Hustler coils are actually a bit higher than the big ones, due to the large end caps on the big coils. That part we agree on. However, if you measure the Q of the Hustler coils statically (on the bench), they don't even come close to your guesstimate. Once the coil is mounted with in the antenna's structure, the Q drops. How much is drops can't be measured. You can come close using the methodology proffered by Jack Belrose, but alas, no cigar.

As for the Hamsticks, the best one is the 10 meter one, because it is essentially a full 1/4 wave. The Q of the 80 meter version is about 20. If you doubt that premise, then do the calculations using the Handbook formulas.
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W5DXP
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« Reply #8 on: July 28, 2010, 01:26:42 PM »

As for the Hamsticks, the best one is the 10 meter one, because it is essentially a full 1/4 wave.

Please forgive a deviation from the central topic but: Any short resonant mobile antenna is electrically 1/4WL long because that's the only way that there could be a purely resistive feedpoint impedance. That impedance on the standing wave antenna is

Zfp = (Vfor-Vref)/(Ifor+Iref)

where Vref is shifted by 180 degrees from Vfor and Iref is shifted by 360 degrees from Ifor.

A resonant 75m hamstick is electrically 1/4WL long and so is a resonant 10m hamstick. The difference in performance is the I^2*R losses in the coils.
--
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.
AA4PB
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« Reply #9 on: July 28, 2010, 01:40:29 PM »

Yes, but the 10M Hamstick is appoximately a physical 1/4 wave long as well as being a 1/4 wave electrically. That means little or no loading coil thus little or no loading coil loss. Its why the 10M Hamstick is so much more efficient than the 75M Hamstick.

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WX7G
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« Reply #10 on: July 28, 2010, 02:46:11 PM »

The problem is Dave, you cannot rely on base impedance measurements. The Q losses, radiation losses, ground losses, and conductor losses are in series with one another. Guess one, and you're guessing the all!

You are correct that we have coil Q, radiation resistance, ground loss resistance, and conductor loss equaling base resistance at resonance. The measured base resistance is 25 ohms. Therefore none of the four variables can exceed 25 ohms. The loading inductor is 48 uH and at 7 MHz is +j2100 ohms. 2100/25 = 84. This is the lowest possible Q the coil could have and that is if the other three variables are zero ohms, which they are not.

We do know the value of the other three variables. The radiation resistance is 3 ohms. That is easily calculated or simulated. The conductor loss is less than 1 ohm. Let's call it 0 ohms. The ground loss has been measured as 5 ohms at 28 MHz and 7 ohms at 14 MHz. For now let's call it 7 ohms at 7 MHz.

Coil loss = Rbase - Rgnd - Rconductor - Rradiation = 25 - 7 - 0 - 3 = 15 ohms. This is a coil Q of 140.

Now insert 0 ohms for Rgnd as it cannot be lower than that. Now the coil Q is 95. And we know that Rgnd is not 0 ohms.  

The Q of 60 measurement you cite is incorrect. Accurate inductor Q measurements are not easy to perform. Yes one can connect a meter to an inductor and get a number. But it's not necessarily a good number. One does not simply set a large inductor on the table, measure it, and call it good. It's a number, but is it a good number? How do you know? It must be compared to a calculated value. If measured and calculated do not agree reasonably either the measured value is bad, the calculated value is bad, or they are both bad.  

One method - and that is the method used here - is to measure in situ; in the actual configuration. We measure 140 in situ. We calculate 175 for the inductor without end caps. This tells us that coil Q most likely lies somewhere between 140 and 175.

If you object to these numbers please provide a mathematical proof to back up your theory. Your task is to explain mathematically how an inductor having a Q of 60 (Rcoil = 35 ohms) can inhabit a 7' vertical that exhibits a base impedance of 25 ohms.
« Last Edit: July 28, 2010, 03:48:59 PM by DAVE CUTHBERT » Logged
K0BG
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« Reply #11 on: July 28, 2010, 04:21:29 PM »

I don't know where you're getting your figures from. That notwithstanding, if you have a decent Q meter, like the HP 4342A, it is relatively easy to measure the Q of an RM40. It is less that half the figure you state it has to be.
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WX7G
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« Reply #12 on: July 28, 2010, 05:25:37 PM »

Yes one can easily measure Q with an HP 4342A. But just what are we measuring the Q of? Here is a first-order model. There is more to it - second order effects - that further lower the Q. The circuit consists of the inductor and the test leads connecting it to the HP 4342. These test leads add a shunt capacitance across the inductor.

In the case of the Hustler RM40 inductor the model of the inductor is 48 uH in series with 15 ohms. And there is 1.3 pF in parallel with this. This is the effective winding-to-winding capacitance. The Q is 126. Now let's add 5 pF of test leads. Now the model is 48 uH in series with 15 ohms. And there is 6.3 pF in parallel. The Q is now 58! What decreases the Q is increased circulating current caused by the 5 pF test leads. This increases the current though the 15 ohm Rcoil thereby increasing the I squared R loss and lowering the Q. 

So how do we measure the Q of the RM40 such that the measured Q is the same as when it's part of a monopole? We make it part of a monopole and measure it in situ via an indirect measurement.
« Last Edit: July 28, 2010, 05:39:19 PM by DAVE CUTHBERT » Logged
WX7G
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« Reply #13 on: July 28, 2010, 08:39:35 PM »

Thermal Measurements

Thermal measurements are another way to measure loading coil loss. From that we can calculate coil Q. Temperature measurements can be used as part of a DC-to-AC transfer measurement.

Method:
Apply RF until the coil temperature stops increasing. Then apply DC across the coil and adjust it for the same temperature. An IR thermal probe is used to measure coil temperature. The DC power is easy to measure and is accurate. DC current multipled by DC voltage is the DC power dissipated. This is equal to the RF power dissipated. From this we calculate coil Q and obtain a more accurate number that helps refine the ground loss resistance number. I will plan on doing this measurement this weekend.
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K0BG
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« Reply #14 on: July 29, 2010, 06:09:58 AM »

What follows is an answer to a post in the Articles form, here on eham.net. While perhaps not directly applicable, it nonetheless contains relevant points.

Quote
by WB9JTK

<<I infer that the point in this thread is one mans individual antenna installation. For this one specific antenna installation (a very short monopole, in which there is no mention of any matching network), it seems to me that measuring the SWR bandwidth is a very good indicator of efficiency. >>

Wrong. That is exactly the type of myth that gets us in trouble.

Bandwidth, as N7WS pointed out, is primarily a function of how rapidly the reactance changes in an antenna. This especially includes small mobile antennas.

In a small loaded antenna the amount of loading reactance as well as the characteristics of that loading reactance has a larger effect on bandwidth than the loss resistances in the system.
Bandwidth is all about the amount of capacitance in above and across the loading coil, not a few ohms of resistance here or there.

This is especially true in a mobile antenna, where the fixed ground loss resistances we can't do anything about are a significant part of the feedpoint resistance on lower bands.

For example, if I add fixed capacitance across a high-reactance loading coil by using large metal support plates the inclusion of those plates will always cause the bandwidth to narrow while the effective coil loss resistance INCREASES from the addition of those plates. This is actually common, and it is an effect that reduces bandwidth and efficiency.

The same is true as the loading coil is moved up and down in the antenna to various locations. If the system does not have significant capacitance above the loading coil compared to stray capacitances from the coil to the world outside the coil, we can again have a case where bandwidth narrows and efficiency suffers greatly.

If we leave the coil in one place and add a capacitance hat high above the coil near the tip of the antenna, we INCREASE bandwidth greatly while increasing efficiency.

This also applies to simple tall structures, like 1/4 wave tall verticals. If the structure is thick throughout its length it can have very wide bandwidth even though efficiency is good. For example a typical 160 meter vertical made from one foot face tower will cover from below 1800kHz to above 2000kHz with good SWR and excellent efficiency, while thin wire will have slightly less efficiency and much less bandwidth. If we make the antenna thick at the open end and narrow at the feedpoint the feedpoint resistance can increase, the bandwidth can increase, and the efficiency increase.

Despite the popular myth that circulates we cannot look at bandwidth and use it as a guide to estimate efficiency. That idea is false with many antennas, including small mobile antennas.

<<Another important thing is that the gentleman that started this thread has proved that 'amateur radio experimentation' is alive and well... and a great way to learn.>>

Only if we do something in a way that allows us to learn, or we have some core understanding of what we are looking at and how it affects what we want to know.

We have to measure what we want to know. If we measure the wrong thing and then leap to a false conclusion about something we never measured, we can actually be getting less educated through experimentation. This effect is common in many areas.

We have to use good methods, and that generally means we measure what we are trying to define and confine our results to that area only.

For example if I only record and measure SWR and fiddle around with the antenna, the only thing I can learn from my experiments is what I did to SWR. I can't reach any conclusions about efficiency because I never measured efficiency. I can give examples of how efficiency can increase, stay the same, or decrease when bandwidth changes or feedpoint impedance changes.

We should wipe the false idea from our minds that feedpoint impedance indicates efficiency, or that a log full of contacts means our antenna has high efficiency. It's very tough to judge a ten-fold change in antenna efficiency based on a log book.

I've worked as far as Australia on 160 meters with a few hundered watts using an antenna with less than 1% efficiency, and I've worked 5 watt stations from Japan and Russia on 160 using very poor antennas. If long distances can be done on 160 on a regular basis with low antenna efficiency and low power, I'd expect DXCC to be easy on 40 meters and higher even with a fraction of a percent antenna efficiency.

Like bandwidth, the log is next to meaningless.

73 Tom
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