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Author Topic: The mobile antenna is an inefficient radiator  (Read 1287 times)
G0GQK
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« on: November 15, 2003, 12:34:52 PM »

I was looking through an old copy of the Radio Communication magazine and it was interesting to read a few facts about mobile antennas. The following figures are typical performance figures for a centre loaded antenna, properly earthed to the vehicle.

On 10 metres there is a 63% efficiency figure, the gain is -2dB, so if your power meter is showing a 100 watts output, 63 watts is being radiated.

For 20 metres the antenna is less efficient, the gain is -20dB and with the power meter showing 100 watts the effective radiated power is 10 watts.

The antenna gain on 40 metres is -14dB with an efficiency figure of 7%  so that on a 100 watts only 7 watts is going out into the ether. If you find that your antenna isn't quite resonant and you "tune" it with your rig ATU you'll lose even more radiated power !

When I'm coming at you with 40 watts SSB on 20 metres and you give me a 5/7 report your recieving all of my 4 watts !

OK, buy yourself a 400 watt amplifier and then on 20 metres you'll be socking them with your 40 watt signal. Wow !

73, Mel
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K0BG
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« Reply #1 on: November 15, 2003, 07:47:53 PM »

Depend on which antenna you are comparing to which standard. Personally, I like the comparison to a full size vertical (1/4 wave) over a near perfect groundplane as 100%.

On 160 meters (metres) 2% or 3% it about the best there is. On 10 meters, you can approach 80% or so as you still have some ground losses to deal with.

Size manners! Small loading coils, short lengths, and poor mounting techniques all add insult to injury in the efficiency department.

However, you're correct, under the best of band conditions, a few tenths of a watt is all it takes. Makes you wonder why all of those base station operators with large antenna arrays always run too much compression and high power 24/7.

Alan, KØBG
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AA4PB
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« Reply #2 on: November 15, 2003, 08:36:48 PM »

OK, buy yourself a 400 watt amplifier and then on 20 metres you'll be socking them with your 40 watt signal.
------------------

The only problem is that the antenna exhibits the same inefficiency on receive as it does on transmit so the other guy also needs to be running 400 watts to make things equal.
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Bob  AA4PB
Garrisonville, VA
G0GQK
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« Reply #3 on: November 16, 2003, 03:21:25 PM »

The standard for evaluating the mobile antenna figures was as outlined, a centre loaded vertical antenna with the most efficient coils,( not bucket sized coils,)  a quarter wave length long, and properly earthed to the vehicle. As the figures were collated for the Radio Society of Great Britain, RSGB, there can be no doubt whatsoever that the figures are correct. As it would be impossibe to evaluate radiation figures from a perfect 100% ground, even if one existed in a mobile transmitting situation, any results in that matter would be erronious.

As a matter of further interest, on 160 metres the gain on a quarter wave mobile vertical would be -26dB with an efficiency of 0.25% and with a transmitter output of 100 watts the radiated power would be 240 mW
On 80 metres where the efficiency is 1% the radiated power is of course, one watt.

73, Mel
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AC6DN
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« Reply #4 on: November 17, 2003, 05:17:00 AM »

Well it all relative. First of all, I think your numbers are amend at Ham sticks, Hustlers, or alike. Still I think the average mobile ham antenna it probably –20 or so on 80M…
However, if truly want to make a comparison, worst case. Lets say a home stn ¼ wave gnd mounted vertical, with 200, 50 ft radials (what ever the perfect ground sys. is?). Vs. a typical Hustler antenna, mounted on the bumper, with no attempt to make a, really good, gnd (like most ham do), on a small car. Thin, I’d say, your number are vary conservative (low)…

Bruce
AC6DN
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K0BG
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« Reply #5 on: November 17, 2003, 10:49:48 AM »

Mel, I haven't seen the information you're speaking about, but from the figures they are about right for the run of the mill commercial mobile antenna. Optomizing all aspects, you can achieve a little better than you listed. 1% or even 2% is possible on 160 if you use a long enough antenna and a large enough tophat. This said, most mount their antennas (even good ones) so poorly, I suspect (as one poster stated) the loss figures are much worse in reality.

Alan, KØBG
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AA4PB
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« Reply #6 on: November 17, 2003, 03:25:39 PM »

1% or even 2% is possible on 160 if you use a long enough antenna and a large enough tophat
----------------

Probably even better than that if you make the whip 130-feet long and have a big enough vehicle :-) HF mobile antennas are all about trade-offs and where you are willing to make them. I'd be afraid to drive a car with some of the antennas that win the shoot-outs.
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Bob  AA4PB
Garrisonville, VA
KA5S
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« Reply #7 on: November 22, 2003, 04:10:41 AM »

Jerry Sevick, W2FMI, has a slim little book called _The Short Vertical Antenna and Ground Radial_. At its end he has some things to say about the Hustler(tm) mobile antennas. Their coils had more loss than just about everythng else he reports on.

However... "On 20 meters, even the hustler(sic) whips are only pooorer by 5 dB from the ideal 1/4-wave condition ... the question comes up as to whether it's worth replacing the Hustler resonators with high-Q coils. There would be only a 2 dB improvement on 40 and 80 meters...."

I scramble wound some hundreds of feet of wire onto 4 inch PVC pipe about 15 years ago and built a base loaded 160 meter mobile antenna. I worked 90 stations in 35 states during a September 160 meter CW weekend, including some on both coasts, from near Fort Worth. Texas. Probably 0.5 percent efficient. Maybe less.

If it works, USE it.

Cortland
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