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My primary vehicle is a Smart car, with a roof that's a piece of plastic(100% moon roof). Not the most awesome of cars to use for grounds. I'm building a bumper mounted antenna tower, EMT with some square tube. Earlier today I stumbled across an article on 2 meter slot antennas cut in to satellite dishes. Literature is repetitive either directed towards that subject or TV and UHF broadcasting. I like the idea of a Dipole analogue that doesn't look like an antenna... Currently the 1/2 wave whip makes it look MORE like a cheap RC toy...

This isn't a basic theory question, more of a practical application...

So the questions are:

Would a Slot antenna cut into a piece of 1.5 inch wide, 1/8 inch thick bar stock work? Minimal expense and I can get it almost anywhere.

Would that be better than just cutting the slot into the bars? I can't find out if that's very directional, I assume I'd cut it in the top and use weep holes, cheapest option.

Any better Ideas?

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    $\begingroup$ For those that are going huh, a slot antenna is virtually, electromagnetically a dipole turned 90 degrees on axis. So a slot antenna horizontally across my car is close to a vertical dipole in radiation. This allows for some really stealthy things. $\endgroup$
    – Omagasohe
    Feb 15 '18 at 21:38
  • $\begingroup$ What other design goals do you have other than "Not the most awesome of cars to use for grounds" and "makes it look MORE like a cheap RC toy" ? $\endgroup$
    – Edwin van Mierlo
    Feb 16 '18 at 9:57
  • $\begingroup$ it is done before on a Smart Car : youtube.com/watch?v=BmEaPVrXyWU $\endgroup$
    – Edwin van Mierlo
    Feb 16 '18 at 10:00
  • $\begingroup$ @edwin I'm aware that I can put an antenna on a smart, I currently have a 1/2 meter whip. I still want to open the rear hatch, his HF antenna mount makes it impossible to open the lower half. The frame I'm building will go up and over the rear of the allowing for an easy return to stock and full use of the rear cargo, I actually use the smart for transporting things. for the foreseeable future I'm going to have only a 2m rig. Since i'm welding anyway I wanted to know if anyone had knowledge of if it'd work. $\endgroup$
    – Omagasohe
    Feb 16 '18 at 12:00
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    $\begingroup$ A slot is a hole in a sheet, not a hole in a bar. You can get anything to radiate, but a long, thin slot in a long, thin bar doesn't sound like a slot antenna. $\endgroup$
    – Phil Frost - W8II
    Feb 16 '18 at 16:17
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This was published in the March 2018 QST. Maybe this or a variant would work for you

http://www.w6nbc.com/articles/2018-03QSTdesigncompetition.pdf

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  • $\begingroup$ that is is not really a "slot" antenna. It is an extremely short vertical radiator with a large capacitor ... personally I do not believe that this is a very efficient antenna. $\endgroup$
    – Edwin van Mierlo
    Feb 16 '18 at 9:58
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So you don't want a whip, as it makes the Smart Car look like a "RC Toy" (although some in the hobby, me at least, will disagree, afterall antennas on cars are fantastic)

And you don't want to hinder and doors/trunk/whatever opening, as you use the vehicle for certain purposes... (fair enough)

And you cannot use any mag-mount, as the vehicle is largely plastic, which also means you don't have a "ground plane" or whatever name you want to give it. (Technically the metal horizontal parts of a vehicle will act as a capacitor to earth when properly bonded. But some think the car itself is the ground plane)

You are thinking of a "slot antenna" but just to realize that a "slot" in the "slot antenna" is the same size as a 1/2 wave dipole, the "slot antenna", besides that it will be directional (not good for mobile use) will be quite large... which you do not want (the "RC toy" effect again)... as well as difficult to mount. If you have already a problem with 1/2 wave whip, then the slot will be even bigger.

Furthermore, it will be doubtful if you can actually use a slot antenna for VHF, as these are mostly designed for much higher frequencies.

If you don't mind doing a bit of DIY: you can get antenna's for the 87-108 MHz which you can stick to a windscreen (front or back), example:

enter image description here

You will have take out any active/passive components, and tune it for 2m. But it will not obstruct doors, it will not have a visual impact.

Efficiency and how this works is hard to say. I have not done this (yet).

To answer your specific questions:

Would a Slot antenna cut into a piece of 1.5 inch wide, 1/8 inch thick bar stock work? Minimal expense and I can get it almost anywhere.

That is not how a slot antenna works, please look up some designs online. Examples:

You will quickly find that your approach is too simplistic to produce a working model.

Would that be better than just cutting the slot into the bars? I can't find out if that's very directional, I assume I'd cut it in the top and use weep holes, cheapest option.

As per previous answer, the design is much more complex. (and) Yes, a slot antenna is a directional antenna which is not suitable for mobile use.

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  • $\begingroup$ someone down voted: would you like to share why ? (so I can learn and imrpove my answers) ... thanks. $\endgroup$
    – Edwin van Mierlo
    Jul 22 '19 at 13:26
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On a vehicle with a steel body and roof, the most feasible 2 meter slot antenna design is a pair of square flat plates with spacers to maintain the space between them. The slot ends up being the horizontal space between the plates. The top plate is connected to the center of the coax feed line, and the ground is the bottom plate to which is joined the braided conductor of the feed line. This can be magnet mounted to the roof by placing the magnets on the underside of the bottom plate, and then placing that side down on the roof.

Also it is important to insulate the top from the bottom plates in the corners where they are spaced apart. One method is to use a tubular plastic spacer of the proper length, and using short self tapping screws that only go into the plastic - do not use full length bolts, washers and nuts as they will provide an electrical connection between the plates that will short the antenna.

This particular design was explained in QST - you will have to search for 2 meter slot mobile antenna as I do not have the exact volume information.

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  • $\begingroup$ Hello and welcome to ham.stackexchange.com! $\endgroup$
    – rclocher3 ♦
    Aug 13 '20 at 16:44
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    $\begingroup$ Re-commenting after reading the QST article - it is really describing a Planar Inverted F Antenna (PIFA) - well known in mobile devices - not a slot. A slot, as the counterpart to a dipole, is defined by having a resonant length, a current maximum at the ends, a voltage maximum in the middle. The PIFA will still work, it's low profile and easy to match. It tends to radiate upwards more than a monopole. $\endgroup$
    – tomnexus
    Aug 13 '20 at 21:34
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Check out this video from AG6IF who built a 2m slot antenna out of an old parabolic dish. His application was for home mounting a stealthy antenna, but the interesting notion there is that the slot is curved. His video shows a chart from an analyzer as well.

This indicates to me that it's worth trying your idea too.

Hope this is helpful!

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It's a little late of a response, but my answer is try your idea of the slot cut into the bar stock and see if it works. Try it with a known repeater and turn the antenna to see the null off the side.

Try bending it into a hoop like a vhf squalo and see if that works. What is there to lose?

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