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[Articles Home]  [Add Article]  

Mobile Grounds

from Alan Applegate, K0BG on February 8, 2018
View comments about this article!

"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: 10/18/2006





Mobile Grounds

I keep seeing the same misconceptions over and over on these pages about grounding mobile antenna mounts, and the chassis of the mobile transceiver in question. It is touted as an absolute cure to every mobile malady from high SWR to ignition noise reduction. It isn't! So lets compare the mobile set up with a fixed station one, in an effort to clear up this misconception.

It is easy to set up an transceiver, and feed it a nominal 12 volts DC from a battery. A chunk of coax run up to a resonant dipole, and we have near instant communications capabilities. Does this set up need to be DC grounded to work? Absolutely not! Would you have to ground the chassis of the radio to make it work? Absolutely not! Doing so might afford some level of lightening protection, but as far as communications goes, it is not necessary.

If we were to remove the side of the dipole that is connected to coax shield, and string the remaining wire straight up, we have a vertical antenna without a ground plane under it. It obviously wouldn't work too well as the coax cable itself would be the only return path the RF would have. This is not an ideal situation to say the least.

We could pound in a ground rod, connect the shield to it, and our vertical would work a little better perhaps, but what it really needs is a radial field. Not just a few radials, but a lot of them if we want a good level of efficiency and minimal problems with RFI. Neither the radial field nor the transceiver need be DC grounded to work efficiently.

When mobile, instead of a radial field we have the body of the vehicle, and what ever amount of capacitive reactance there is between it and the surface under it. The net result is, there is ground loss. Depending on the vehicle, the surface under it, how the antenna is mounted, and what frequency we're operating on; this loss is from 2 to about 20 ohms, but can be more. This induced ground loss reduces efficiency in a similar manner to our aforementioned vertical wire without a proper radial field. Thus, it is not uncommon to have efficiencies of less than 5% on the lower HF bands.

Improper mounting will cause additional losses. For example, mounting the antenna too close to the body. Although doing so is not really ground loss per s (it is stray capacitance loss), the net result is the same; less efficiency. Since mobile antennas are already a big compromise, it behooves us to keep the losses we have control over as low as we can. One way to reduce the losses is by properly mounting our antennas, and adequately bonding the various bolted on pieces of the vehicle.

Lots of amateurs are unwilling to drill holes in their vehicles, and the reasons are quite varied (but this is not the subject at hand). These folks make do with trunk lip mounts, license plate mounts, and all manner of trailer hitch mounting schemes. All well and good. But somewhere along the line, the idea that grounding these types of mounts with a wide braid to a chassis hard point will magically make them as efficient as center-of-the-roof mounting. It won't!

In all fairness, the coax shield should be connected to the body of the vehicle, and close to the base of the antenna. A ground strap might improve this connection, but it won't do anything to improve the ground losses encountered! And it won't magically cure RFI egress from the various electronic devices in the vehicle! And it won't magically cure RFI ingress to those same electronic devices! If you installed a ground, and it solved a problem (RFI or other wise), then something else in your installation is (was) amiss.

It is best to think about a mobile antenna, and the vehicle it is mounted on, as a system. We already know we have an inadequate ground plane, and we do not have the option of running radials hither and yon. This leaves us with very few options. A better mounting location and/or method, better bonding, a higher Q inductor (to a point), increasing the length of the antenna (to a point), or perhaps a cap hat more or less sums them up. Anything less is exactly that, less!

By the way, efficiency has no relation to how many DX stations you've worked, or how low the SWR is. From a mobile standpoint, about all they prove is, QRP is alive and well!

DC Wiring & Grounds

There are misconceptions about DC wiring too. In a mobile installation, any requisite DC ground is supplied adequately by the power cable that came with the transceiver. Adding a ground to the chassis isn't all bad, but if it solved a problem (RFI or other wise), then something else in your installation is (was) amiss.

In fact, there are instances where hard grounding the chassis can cause you grief. For example, if there is a differential in current flow between the ground side of the power cable and the chassis of the radio, you have what is commonly referred to as a ground loop. This problem occurs most often when body-on-frame vehicles are not properly bonded, and/or the DC power wiring is incorrectly installed.

Part of the problem is the surfeit of anecdotal information across the web. For example, I recently read an article where the author suggested twisting the DC power cord in as many twists as possible using an electric drill! This technique is supposedly the cure for alternator whine, and a host of other mobile maladies. This is an absurd notion without any merit. I might add, using RG8 coax as power cabling is as inane. The truth is, induced and radiated RFI are best cured at the source.

Related Problem

If my e-mail inbox is any indication, the most prevalent problems are those related to coax, and coax connectors. I have information on my web site about properly installing PL259s. There's a ton of information other places on the net about them too, including these very pages. The ARRL Handbook also explains the procedure in detail. So does information from the manufacturers themselves (primarily Amphenol). Why then is this still a problem?

Adding insult are screw on and crimp on connectors. While they might be okay in some installations, vehicle vibration makes their use suspect.

A typical HF mobile installation has a transceiver, wattmeter, antenna controller, perhaps an amplifier, and an antenna. That's about eight coax connections. If even one of them becomes intermittent, you're going to have problems.

Do yourself a couple of big favors. Buy silver plated connectors, not the "japanned" or Astrofinish ones because they are nearly impossible to solder. Don't buy connectors with white, Teflon-look-alike insulators. They're not Teflon, they're polystyrene (or polyester), and they melt! Silver plated, TFE insulated, PL259 connectors cost about $6 each. If you paid a lot less, you got a lot less.

The best advice I can offer about PL259 connectors is this; take whatever time it takes to install them correctly. Using the proper tools, it takes me about 3 minutes to complete a PL259 without a reducing sleeve (RG8), and about 5 minutes with one (RG58 or RG8X). One of my good friends can't do one any faster than 15 minutes. The point is, it doesn't make any difference how fast you do it, but how good you do it that counts.

Here's a piece of trivia. Circa 1930, an Amphenol engineer named E. Clark Quackenbush, designed the UHF coaxial connector. We commonly call it by its military designator, PL259. I bet he'd turn over in his grave if he knew the grief he has caused us all. If you want a little more history about UHF and other types of connectors, go to the Marvac web site.

Conclusion

We're in the age of Instant Gratification, where anything will do as long as some communication is possible. If this is your situation, and you're willing to live with it, fine. I'm not. I prefer to plan well, execute precisely, and operate moderately. While this takes more time, the rewards are much greater.

Alan, KBG
www.k0bg.com

Member Comments:
Add A Comment
 
Mobile Grounds Reply
by N7ZXP on February 8, 2018 Mail this to a friend!
Great artical. Thanks for putting it out.
 
Mobile Grounds Reply
by K8QV on February 8, 2018 Mail this to a friend!
I know that "instant gratification" crack was meant to be derogatory, but that's taking the rather narrow view that "if he's not doing as I do then he is wrong."

We have several aspects to this hobby. One would be using your radio on air and another is building, tweaking and optimizing to the extreme. Doing "perfect on paper" installations is a worthy endeavor and fun for some people. So is using your available time actually making contacts on a radio.

Years of personal experience has shown me that my various mobile set ups with mag mounts and Hamsticks (both short and full size) equal and occasionally surpass the performance of guys with "perfect" mobile installations. Clearly, the guys with holes in their car bodies and miles of grounding strap beat my efficiency specs, but attaining the best efficiency isn't my goal and I won't waste my time or money doing that when I can so easily compete with their actual performance results.

As they say, your mileage may vary.
 
RE: Mobile Grounds Reply
by KB6QXM on February 8, 2018 Mail this to a friend!
I am glad to see that eham is republishing some of the older and better articles. The bad news is that some of the links in the article do not work anymore, but that is to be expected with an article this old.

The technical aspects will not change.

Unfortunately in my state of taxes and regulations, mobile ham radio is not a viable option. I thought about it because of all of the HOAs, CC&Rs and "visual impact" proponents as an option to do ham radio without anyone complaining, but again in the wonderful state of California, that option was taken away due to the "distracted driver" laws.

If our one amateur radio organization would do anything above and beyond pushing contests, maybe they could get an exemption or clarity for amateur radio operators, such as VOX only while driving.

If I move out of California, then maybe I can experience the enjoyment of being stuck in traffic and working DX or being in a rag-chew at the same time.

73
 
Mobile Grounds Reply
by K6YE on February 8, 2018 Mail this to a friend!
Alan,

It is very nice to revisit a great article. We moved to a new residence here in California and until the tower, yagis, and sloper are erected, I am using a Tarheel antenna on my Ford Ranger.

Miss seeing you at the various functions in Colorado. Keep up the great work.

Tommy - K6YE
DX IS and CW RULES
 
RE: Mobile Grounds Reply
by GM1FLQ on February 9, 2018 Mail this to a friend!

"I know that "instant gratification" crack was meant to be derogatory, but that's taking the rather narrow view that "if he's not doing as I do then he is wrong.

.....Doing "perfect on paper" installations is a worthy endeavor......

....... I won't waste my time or money doing that when I can so easily compete....."



Yeah kind of reminds me of.....once upon a time, a long long time ago, some real nasty people with nasty narrow views asked that pupils get the basics right - count, read, write and spell properly.

Then one day, along came some very nice people with big wide "blue sky" views and said that wasn't really important, just have fun.


The pupils and many of their parents were all very happy & liked these nice people very much - because now every illiterate and innumerate Tom, Dick and Harry could go to university & leave with useless degrees and all be happy as a pig in the proverbial.

.........................living happily ever after in a great big sea of mediocrity.


I'm sure all those with the big wide views will easily have enough width to accommodate another view.




 
RE: Mobile Grounds Reply
by K9MHZ on February 9, 2018 Mail this to a friend!
Where do you come up with this stuff, OM? I thought the thread was about mobile grounds. Not enough sunlight there in the UK?

Good grief.
 
Mobile Grounds Reply
by KK6HUY on February 9, 2018 Mail this to a friend!
Good article, but if you're going to be putting any kind of radio in a modern vehicle you'd better know what you're getting yourself into with regards to the airbag system.

Yes, the best place to put an antenna is dead center on the roof. Well, that's literally right on top of my airbag detonator cables for the side-curtain bags. It is in most modern vehicles. I REALLY don't want to be leaking any stray power in that position or I may have some loud bangs followed by a totaled car.

There's a reason cop cars have all the transmitting antennas on the back of the vehicle. It's not because the radios work better that way.

Be safe!
 
RE: Mobile Grounds Reply
by GM1FLQ on February 9, 2018 Mail this to a friend!

"Where do you come up with this stuff, OM? I thought the thread was about mobile grounds."

Well I thought that as well, but '8QV launched straight in about the "derogatory" narrow view of the author.

He clearly wanted to deviate from mobile grounds to narrow views, then to how good the that'll do approach is (funny you didn't notice) - so I just thought let's deviate then.

Happens on forums you know, start one place and meander off somewhere else. Terribly sorry though, didn't realise you were chairing this one......
 
Mobile Grounds Reply
by AC5WO on February 9, 2018 Mail this to a friend!
Great article except for the recommendation for soldering PL-259 connectors onto cables. No RF professional in the 21st century uses a solder connection to the coaxial cable braid because the heat of soldering is going to melt and deform foam polyethylene dielectric of common LMR-type low-loss coaxial cables plus the inner surface of the inner foil layer, not the braid, is the RF signal path. The PL-259 is a horrible connector design both in terms of RF performance and assembly difficulty. Buy or borrow the proper crimping tool and use connectors with a crimp ring connection to the braid.

I save scraps Teflon coax like RG-142 to make jumper cables. Teflon cable will survive the heat of soldering, even when using the chunky prehistoric PL-259/UG-175 reducer combination.
 
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