Antenna Grounds when mobile

Started by Geek, June 16, 2013, 12:05:26 pm

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Geek

For those of you who are engaged in mobile or portable HF communications, what are you using for grounding?

WA4STO

Quote from: Geek on June 16, 2013, 12:05:26 pm
For those of you who are engaged in mobile or portable HF communications, what are you using for grounding?


As you likely know, there's two kinds of 'grounding'.  In the case of mobile operation, the big problem is the type of 'ground' that makes up the antenna ground plane. 

Think of a dipole for 40 meters.  One half of it is about 33 feet long.  The other half (the 'ground plane' half) is also 33 feet long.

Now, where in the world (literally) are you going to find 33 feet of metal on a car body? 

So antennas at HF frequencies become a terrible problem.  Everything is a compromise there, meaning that mobile signals are going to be much much weaker than the signals from your fixed location.  Bummer.

Here's how one guy attempts (very successfully) to overcome the laws of physics when dealing with mobile antennas.  His callsign is K0BG and he is known around the world for his efforts. 

http://www.k0bg.com/



73

Luck, WA4STO

Geek

I am learning about the problems with HF antennas and it does seem to be a challenge.  However, we have guys here going on camping trips and communicating hundreds of miles, granted via CW.  So I am wondering what do they use for a ground?  They obviously aren't pounding copper rods into the ground at every campground.

Quietguy

June 16, 2013, 04:49:34 pm #3 Last Edit: June 16, 2013, 04:55:23 pm by Quietguy
As Luck mentioned, there are two kinds of grounds we are interested in - "earth" grounds and antenna ground planes.  Earth grounds are what you think of when you talk about power safety grounds or lightning protection and can be accomplished by driving ground rods into the ground.  However, these generally make poor ground planes, which are what you want for antenna systems.  Earth grounds are not required for antenna systems*, and a driven ground rod makes a pretty poor ground plane unless you have a bunch of them.

Let's take Luck's example of a dipole a little further and turn it vertical.  Now let's keep it vertical but lower it until the feed point is at ground level - that places one half of the dipole laying on the ground.  If we adjust the length of the lower section to compensate for the effects of laying on the ground instead of being up in the air, we call it a counterpoise.  If we add a bunch more of them, and stretch them out equally spaced from each other, we call them radials.

Now if we take the vertical part and angle it over to tie the end to a convenient tree we have a sloper - and that and a dipole are the two easiest antennas to use in the field.  If you use a sloper you have a counterpoise of the correct length for the bands you are operating stretched out on the ground.  If you have a dipole, the "counterpoise" is up in the air.

We here in the US get criticized by the rest of the world for using the word "ground" because of the confusion it creates.  The rest of the world says it should be "earthed" if you want a driven rod, and what we call "chassis ground" (the reference point in a circuit) should be called "zero volts".  I don't know what they use for "ground plane" for antennas - but the point is we use the word "ground" to describe three separate concepts and it causes confusion.

Antenna ground planes are created with wires, not rods, and they can be elevated or buried.  That means that the ground plane of a mobile antenna is simulated by the metal mass of the vehicle... which accounts for their generally poor performance.

* - Edit to add:  Unless you want to incorporate lightning protection, which may be of interest to some members of the group.

I hope my wordiness doesn't just cause further confusion...
Wally

WA4STO

Quote from: Geek on June 16, 2013, 04:20:24 pmHowever, we have guys here going on camping trips and communicating hundreds of miles, granted via CW.  So I am wondering what do they use for a ground?  They obviously aren't pounding copper rods into the ground at every campground.


My wife and I spent a number of years full-timing in our RV.  And, yes, hundreds even thousands of miles with a decent mobile antenna can be realized.

Follow K0BG's advice and you'll go far.  <sorry, couldn't resist>

73

Luck, WA4STO

Geek

So let's take a dipole antenna.  Half is supposed to be vertical and half is operating as a ground plane.  What is the orientation of the half operating as a ground plane?

Alternatively, if you have an end fed antenna, like the LNR Precision models, there is a ground screw.  What do you connect to that and how should it be oriented?

Another alternative would be a vertical antenna that goes on a tripod.  What is required for that?

gil

Well, a dipole is typically horizontal. If you make an "L" with one leg on the ground, you point the "ground" wire towards the station you are talking to.. If you can.. This is done mostly using "random' wires, with a quarter wave counterpoise. I don't think the orientation makes a huge difference, but I might be wrong!

I never connected a ground to my end-feds... Works great without it.

Gil.

KC9TNH

Quote from: Geek on June 16, 2013, 10:01:26 pm
So let's take a dipole antenna.  Half is supposed to be vertical and half is operating as a ground plane.  What is the orientation of the half operating as a ground plane?

Alternatively, if you have an end fed antenna, like the LNR Precision models, there is a ground screw.  What do you connect to that and how should it be oriented?

Another alternative would be a vertical antenna that goes on a tripod.  What is required for that?
Geek, in your OP you also mentioned portable (vs. mobile). The portable issue is very much a "what do you want to do & where do you have to do it & what do you have to do it with?" type thing.

But looking at the above, as Gil said, your dipole is oriented (usually) horizontally, both wires. Think of one wire as a return for the other; hence it's also in balance, a true dipole anyway.

On an end-fed, there can be lots of variables, including overall length, height above ground (both points), length & quality of the feedline, quality of the ground (the dirt, the earth) and use (or not) of a counterpoise and/or ground.  Here is where it can seem rather less like science and become part black art.

Your end-fed may function with that other screw having a connection to a wire or wire array coming away from the radiating leg of the antenna and be splayed along the ground, or multiple ones in different lengths depending upon bands you want to work - similar to function of radials to a vertical antenna like a starfish.

You can also work an end-fed running that side of the connection straight to ground (the earth, the dirt).  If you have a VERY long coax feedline and aren't running too much power (hellooooooo QRP) you can run none of that, just the radiator. In THAT case the push back from the radiator is actually being taken up by the shield in the coax. Ultimately it's trial and error for that site/situation and what works.

You can literally run an end-fed sloping up away from your campstool; likely it'll play a bit better if you get the feedpoint end by your coffee-pot UP away from the ground at least 6-8 ft because at low HF freqs it's getting too much interplay from the actual earth and you're not going to see the SWR you want and possibly not get the efficiency out of it. It'll radiate, but so can a mop bucket.

So in general terms -
- dipole, horizontal although there are variations of vertical dipoles, slopers, inverted-V, none of which offer any gain over a pure dipole.  Those variations are usually done due to space constraints. It's balanced, with pretty RF waves travelling back & forth over the wire. As said, one side is the ground.

- Vertical, stuck up in the air, with starfish radials growing out from its base.

- End-fed, just what it says, with some type of counterpoise wire (usually) and rule of thumb on that is 1/4 wave for the band you want to work and that's what's connected to the other side of that connector you're talking about. Don't run it directly under the wire; away from or perpendicular to the wire, or even tossed along the baseboard if indoors is what you want.  The counterpoise will also help dissipate any RF inside the shack if you're running stealth and can't get the feedpoint all the way out of the enclosed space. Even 2.5w of RF bite is noticeable; don't be that guy stepping barefoot onto his own counterpoise.  8)

When you're running an end-fed, see which of the 3 ways (counterpoise, grounded, or nothing) works best for you and try it all 3 ways; it's the only way you'll know. Somewhere in one of the threads here is a pic of my (oops, 'scuse me, YOUR) 817 tailgated up at the hunting property with wire up into a tree & couple of counterpoise wires laid out in the grass. I just brought the counterpoises in closer for the sake of the shot, but maybe a pic is worth something.

Antennas can be a mania unto themselves. Lots of things function; often the work is in actually getting the sucker in the air.
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Geek

Last evening I went to a meeting of a local Ham club.  Like everything else about this hobby, it seems that I was again the youngest guy in the room, and I am a grandfather.  :-)

When I arrived there was one truck in the lot and the fellow had a couple antennas mounted, one of which was a "Little Tarheel II" made by Tarheel antennas.  I asked the owner about it and he said he was pleased and had an auto tuner in the vehicle to handle it.  I wasn't entirely clear on how the auto-tuner worked, but the whole package sounded like as painless an installation as one could want for a mobile radio.

KC9TNH

Quote from: Geek on June 18, 2013, 03:00:36 pm
Last evening I went to a meeting of a local Ham club.  Like everything else about this hobby, it seems that I was again the youngest guy in the room, and I am a grandfather.  :-)

When I arrived there was one truck in the lot and the fellow had a couple antennas mounted, one of which was a "Little Tarheel II" made by Tarheel antennas.  I asked the owner about it and he said he was pleased and had an auto tuner in the vehicle to handle it.  I wasn't entirely clear on how the auto-tuner worked, but the whole package sounded like as painless an installation as one could want for a mobile radio.
I've heard of the Tarheel which apparently has been serving many for many years. Not sure how his system works though. Yaesu has their ATAS (which may be similar) and it has a big coil out of which raises a whip mast and the tuner (interfaced to the Yaesu radio) drives a simple motor that literally raises & lowers the mast from the larger base unit.

The suggestion to go check out the K0BG website is pretty good. Any acquaintances who run mobile HF and do it well all tell me that the first essential thing on a vehicle is multiple QUALITY grounding points such that they're as good as "part of" the vehicle.

gil

One more word about end-fed counterpoises: I have heard that 0.05 wavelength is a good counterpoise length...

Gil.

KC9TNH

Quote from: gil on June 18, 2013, 04:14:55 pm
One more word about end-fed counterpoises: I have heard that 0.05 wavelength is a good counterpoise length...

Gil.
That's a great point. The "doctrine" is what I posted above, but most of mine that worked well were with remarkably short counterpoises. Thanks.

WA4STO

Quote from: Geek on June 18, 2013, 03:00:36 pm
I wasn't entirely clear on how the auto-tuner worked, but the whole package sounded like as painless an installation as one could want for a mobile radio.


It's downright amazing at what people will do in an effort to maximize their mobile signals.

This is a somewhat larger version of the Tarheel than the one you saw at the club meeting, but it enforces the point that such installations are actually quite difficult to do successfully.



Durned things are more expensive than some of the HF transceivers on the market.  And when you get done, you STILL have a compromise of an antenna.  Cussed laws of physics, anyway!  :(

73

LH

Geek

That one is going to get noticed going down the road.

KC9TNH

One other comment on end-feds & then will go back to ogling the vehicle above. :)

With a sloping end-fed, looking for some multi-band capability, more wire is not necessarily better if you can't get the overall height at the FAR end, appropriate to the wavelength of the wire you're tossing up.

Example:  If a half-wave is good, a full-wave (or even into long-wire length) should be better right? Not if you can't get the far end of the thing high enough.  Just because you have 66' of space available doesn't mean you should toss up a full-wave for 20m if you can only get it 15' high.  Better to just run the half-wave and get the far end up as high as possible and, failing that, the fed-end as high as possible as well.  Otherwise the ground has more negative effect on the efficiency than the length of wire has benefit.  Just my $.02 from playing around with them. This is one of those things that can actually be modeled & pretty much turns out like the model.  An exception would be an NVIS solution that is designed to be lower from the get-go BECAUSE you just want that limited-coverage rising bran muffin effect.