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HF Mobile Antenna Notes

Created by Alan Applegate, K0BG on 2022-10-30

"Editor's Note: Due to the popularity of some of eHam's older articles, many of which you may not have read, the eHam.net team has decided to rerun some of the best articles that we have received since eHam's inception. These articles will be reprinted to add to the quality of eHam's content and in a show of appreciation to the authors of these articles." This article was originally published on: 03/22/2007

 

HF Mobile Antenna Notes

 

I recently wrote an article entitled Benchmarks. Several folks took me to task for stating; The fact is (contrary to popular belief), antennas are NOT reciprocal in their transmit verses receive performance. Apparently, some readers thought I was speaking of gain. I wasn't. In any case, what really counts is the S+N/N (signal plus noise to noise) ratio produced, but that might not be definitive enough in a mobile installation.

 

You often see receiver sensitivity expressed as 10 dB S + N/N (a 10 dB ratio of signal + noise to noise) at some given microvolt level. In a mobile scenario, we might be better off using SINAD, which is a measure of signal quality that includes distortion. Expressed in dB, it is 20 * log of the signal + noise + distortion to noise + distortion (all in volts). However, you measure or express it, you have to receive some level of signal which will allow you to copy another station.

 

As I referred to in the aforementioned article, an antenna's ability to transmit a usable signal, is not equal to its ability to receive a signal. Depending upon the antenna in question, and the frequency of operation, it may have better receive performance, than transmitting performance. In other words, the antenna receives better over any given signal path, than it transmits over the same path. This is particularly true of HF mobile antennas. The main reason is, they don't have gain, they have loss, and sometimes a tremendous amount it. For example, some commercial 80 meter mobile antennas radiate barely a watt out of every 100 watts fed to them.

 

On the receive side, most of the inherent loss can be made up by good front end design. In other words, even signals as low as .1 uV are enough to provide a decent S+N/N ratio, if the front end noise and distortion (by what ever cause) are low. Conversely, that 1 radiated watt might not be able to produce an adequate S+N/N ratio at the receiving station.

 

Contrary to a recent Internet article, it isn't one-way skid which causes this phenomena! In fact, it's the non-reciprocality of transmit verses receive performance that has fostered the belief in the first place.

 

 

There is a line from Robocop II, where the lawyer, Holzgang (as played by Jeff McCarthy), replies to a request for evidence against Dr. Juliette Faxx (as played by Belinda Bauer); "I'm sure I can find it, even if it doesn't exist." That line is exemplary of the anecdotal information on the Internet. No matter how bizarre the belief, you can find Facts to support it. Here's an example.

 

Several months ago, I received an e-mail from a new amateur complaining that I had panned his favorite, all-band, $60 HF mobile antenna. His proof that I was wrong was based on the number of DX stations he had worked, and the antenna's marvelously low SWR. The manufacturer's URL was included backing up the claim of superior bandwidth and efficiency, and most importantly, no matching needed!

 

This brings up the point at hand. Bandwidth and efficiency are not reciprocal. They are, for the most part, inverse functions. And just like the comparison between an antenna's transmit and receiver performance, there is more to the story than meets the eye.

 

It is common to relate the bandwidth edges as the points above and below resonance where the SWR reaches 2:1, usually measured at the transmitter end of the feed line.

 

It is also common to use the term Antenna Q when describing the bandwidth, and quite often confusing it with the loading coil's Q. While loading coil Q can effect the overall bandwidth and efficiency, the terms are not interchangeable.

 

Antenna Q it isn't something we can measure easily from the drivers seat. In other words, you really can't get there just by measuring the SWR alone. Rather, you need the complex impedance measurements directly from the antenna. What's more, the reactive component changes more rapidly than the resistive component, and it's not symmetrical about the center (resonant) frequency. Thus the measured Antenna Q say 10 kHz below resonance, isn't the same as it is 10 kHz above resonance, irrespective of the SWR.

 

The methodology used to match the antenna is also a consideration. For example, if a shorted 1/4 wave stub is used across the antenna terminals as a matching network, the apparent bandwidth will be much greater. This results from the antenna and the stub having nearly opposite changes in reactance with any given frequency change. Matching methods that borrow some reactance from the antenna (LC or CL as the case may be), also skew the measurements. Stray capacitance is another important, and difficult to measure factor. The long and short of it is, using a 2:1 SWR measurement as a means of quantifying bandwidth, Q, or efficiency of an HF mobile antenna is suspect at best.

 

How important is bandwidth? In a mobile scenario it isn't much of a concern on 20 meters or above, but below 20 meters it certainly can be. For example, the bandwidth of an 20 meter mobile antenna of reasonable quality (and properly mounted) will be about 150 kHz, or nearly the whole phone band. The bandwidth of a similar quality antenna on 80 meters will be about 10 to 15 kHz. Bandwidth and efficiency are inverse qualities (all else being equal), but bandwidth is not a measure of efficiency without taking all of the other factors into account.

 

One of the popular methods of measuring the (apparent) efficiency of HF mobile antennas is the Shootout. Typically, this involves comparing your antenna's signal strength (transmit and/or receive) to that of others, by mounting them on one given vehicle. This too is suspect, especially if either of the mounting positions (yours or the test vehicle's) is low to the ground, or at the rear of a van. Add in the fact that an antenna's level of performance is (almost) never the same between transmitting and receiving, and synoptically a shootout isn't much more than a subjective test.

 

A better methodology would be to test each antenna as mounted on it's home vehicle, and compare it's performance with a common antenna (acting as a standard) mounted in the same location on each and every vehicle under test. This requires normalizing the data gathered over a statistically large sampling, but the net results would be more exact.

 

There are many another antenna-related terms which are almost always misused. One of these is Capture Area. Capture Area only relates to frequency and gain. It does not relate to the length of wire used in the construction of an antenna. Yet, the Internet-supported myth continues about short, spirally-wound antennas being superior in bandwidth, efficiency, and capture area compared to any other type. This is pure junk science.

 

Don't be fooled by the ad hype about short antennas being as efficient as longer ones, just because they contain a 5/8 wave length of wire. Physical length is the only factor that counts.

 

If you double the physical length (with in reason) of an HF mobile antenna, the radiation resistance increases by a factor of four, along with a near-parallel increase in efficiency. However, the S+N/N ratio increase may not be as noticeable. Again, just because you can hear the other station, doesn't mean he can hear you.

 

Is there a scenario where the other station can hear you, but you can't hear him? Absolutely! Belaboring the point, S+N/N ratio is everything. Whether it is a result of distortion, RFI ingress, localized band noise, and a myriad of other factors, if there isn't enough signal strength to provide enough S+N/N ratio, your receiver is essentially deaf. His may not be.

 

There are many more misrepresentations to ponder. One of those is an HF mobile antenna's power rating. If it has one, it's true capability is most likely less than advertised. It's also a sure-fire bet the antenna isn't very efficient, regardless of the number of DX stations worked!

 

If you're using a decent HF mobile antenna, you will have to do some sort of matching to get the SWR under 2:1 at resonance. Whatever method you choose, don't set it up in your driveway! The reason is simple. Most driveways are concrete, and contain steel reenforcing. This fact will skew the results. So will trees, manhole covers, and nearby vehicles.

 

People also buy antennas for no other reason than the way they look, their ease of mounting and/or setup; bandwidth, efficiency, or power ratings, notwithstanding. However, justifying that purchase with anecdotal references just proves why there are so many lousy antennas on the market.

 

This reminds me of another Robocop II movie line.

 

Near the end of the aforementioned movie, Robocop (as played by Peter Weller), says to Officer Anne Lewis (as played by Nancy Allen), let's give him what he wants. This is exactly what the Internet is doing; it's giving people what they ask for, but far too much of it is Nuke.

 

Alan Applegate, KOBG
www.k0bg.com
 
KC6RWI2022-11-01
HF Mobile Antenna Notes
Alan is great writer, I've read alot on his site, and of course, I value his opinion.
One disturbing fact that he has mentioned, and I was looking for it, is how much loss there is on a hf mobile antenna.
Its right there, he says an 80 meter antenna with 100watts might be putting out 1 watt.
Even higher bands have tremendous loss, no gain.
But its hard to swallow, to understand your mobile setup is a compromise.