gamma on a mobile ?

[AudioShockwav]

OK, I just remember seeing something about the DMROG a few posts back, and I was to lazy to go read through all the threads....

73
Jeff

 

[MrSuburban]

Not really... the height of the puck won't allow for a proper high power coil's overall size if it's going to be fed at the bottom of the vertical.

The coil ends up being fed into the shaft several inches above the feed point. I would also think the coils proximity to the body metal would impact especially at high power levels.

I don't think it was an accident that Dave ran a capacitor in the ROG along with the grounded coil.

He put a capacitor in there because it is part of the matching network

 

[MrSuburban]

because the coil thingy is too big/tall when designed to accomodate high power. the coil will terminate several inches higher than the feed point.

This is where the theoretical hits the practical road. For everyone who's following along - myself included, it might be easier if you included an actual design of your concept as opposed to just the theoretical "it can work" stuff. I'd like to see how a copper tube coil taps into a 1.5" tall puck and feeds into the bottom of the antenna and still accomodates the power handling requirements of the application. I'm a visual kind of guy - I'm sure I'll understand it better when I can see it.

Real simple ever see a Maco or a ringo circular matching section. There isnt going to be a big coil you only need a little inductance. The problem is people look at ham websites and think the pictures relate to CB antennas.

 

[W5LZ]

And I mat be misunderstanding what Roc1 meant. Are we talking about an impedance matching coil, or a loading coil?
- 'Doc

 

[6.0 Liter]

I think I am getting confused now too, I believe we were talking about some type of simple matching device to bring the impedance of an already resonant quarter wave antenna to 50 ohms......

 

[ROC1]

The drawing is not to scale, but should give a general idea of one type of impedance matching device commonly used on mobile antennas.
The bottom shaft of the antenna goes down through that coil and connects to the mount 'clamping' that 'shaped plate' between it and the mount. The shaped plate just has to be of a size not to short to ground at that connecting point. The matching coil I used on a bugcatcher antenna used a copper plate soldered to the bottom winding of that coil. It worked just fine for a lot of years. Took a while to find the right tap point on the coil for that ground wire, but it's certainly 'doable'.
Power handling? The coil was made with #12 wire spaced about 1/8 inch between windings. It handled 500 watts just fine but that means that the antenna definitely had to be resonant! Depending on the amount of power you plan to use, it would be a fairly safe bet to use large sized wire and spacing. If the spacing ever get's to something like 1/2 inch, it's way over done. If the power was in the 1000 - 1500 watt range, I'd say #10 wire and 3/16 inch spacing ought'a do just fine. (That's a guess at the spacing, and probably a bit 'large'.)
- 'Doc

That's an interesting setup but you've kind of made my point.
6.0 asked if this could be done to handle 2kw
Shockwave said yup do it on the outside feed it between the puck and antenna.
6.0 said cool wonder if I could just tap it into the puck.
I said nope - not tall enough.
You said "I don't see why not".

That picture is competely different than what we were discussing. That is not a coil that starts on the puck and terminates between the puck and antenna... That is a completely different system. An interesting one but different.

I personally don't know if I'd want the tap wire flapping in the wind at 70 and 80 miles an hour and I'd think having the coil plate machined up would not be a trivial process. It's also not very clear where or if the open air coil itself terminates which brings up immediate concerns about wind loading and if the coil is going to moving back into the antenna while the vehichle is being driven and potentialy keyed up. Theory vs practical... you're still on paper Doc. Interesting concept though.

 

[ROC1]

just for the record i'm talking about a matching coil...

 

[6.0 Liter]

Hey ROC1, have you ever used a matching coil on the inside of the vehicle? How did it work? Did it perform successfully? What were the disadvantages? Anyone else for that matter can chime in...............Pro's? Con's?

 

[ROC1]

Hey ROC1, have you ever used a matching coil on the inside of the vehicle? How did it work? Did it perform successfully? What were the disadvantages? Anyone else for that matter can chime in...............Pro's? Con's?

I've avoided it, my system is like yours, pushing almost 2kw and I've got a lot of wiring harness in the headliner of the burb. I don't want to splatter any more rf around that than I already do. The other issue is, I don't run a single antenna system. I have a drive around 102" whip and a multi-element setup for shooting skip. If I bury the matching coil between the headliner and roof it will be good for one but not the other and about impossible to get too. I'm kind of wishing I had an old school suburban I could just use for the big stuff, pull the headliner out and call it good... would make it a lot easier to set things up.

 

[W5LZ]

Roc1,
This isn't a 'paper' concept. This type of impedance matching coil has been in use for a lot of years by a lot of people. It's one that is used with the bug-catcher type antennas for example.
That mounting 'plate' is actually just a piece of stiff copper sheeting. It's cut to a shape that's easily bent to solder onto the bottom of the coil. The coil is common coil 'stock', plastic insulating 'bars' running up the sides holding the coil windings spaced evenly. I can't remember the name of the company, but it's dirt common.
The bottom copper plate/sheet connects between the bottom of the antenna shaft and the mount, and is the 'hot', or antenna connection. The 'tap' goes to ground, usually at the base of the antenna mount. That makes the whole assembly a matching coil between antenna and 'ground'. It does the exact same thing as that 'inside' smaller enameled wire coil, it's just in on the outside. That 'outside' part makes it very easy to re-tune when necessary, especially since it's adjustable (put an alligator clip on the coil end of the wire).
Does that grounding wire flop around in the breeze? Not much, if any. Can it get pulled loose? Yes, it can, just pay attention, you know? Can something else go wrong with it? Sure, just more of that pay attention thingy 'cures' that. I've also seen (and used) a piece of plastic 'pipe insulation' slipped down inside that matching coil (fits around the antenna mast) to 'stabilize' the coil. I've used that matching method for something like 10 years with a bugcatcher on at least three different vehicles, and I'm not the only one who has. So, for a 'paper concept', it's pretty 'real'.

If the coil you described runs/connects between the antenna and the mount and not the 'ground' under that mount, then it isn't an impedance matching coil, it's a loading coil. A completely different animal...
- 'Doc