A Quick And Stealthy Mobile Slot Antenna From Copper Tape

[Ben Eadie (VE6SFX)] is at it again with the foil tape, and this time he’s whipped up a stealthy mobile sunroof antenna for the amateur radio operator with the on-the-go lifestyle.

You may recall [Ben]’s recent duck tape antenna for the 70-cm ham band, an ultra-lightweight design that lends itself to easy packing for portable operation. The conductors in that antenna were made from copper foil tape, a material that’s perfect for all sorts of specialized applications, like the slot antenna that he builds in the video below. In the ham world, slot antennas are most frequently seen cut into the main reflector of a direct satellite dish, often in hopes of avoiding the homeowner association’s antenna police. Even in the weird world of RF, it’s a strange beast because it relies on the absence of material in a large planar (or planar-ish) conductive surface.

Rather than grabbing an angle grinder to make a slot in the roof of his car, [Ben] created a “virtual” slot with copper tape on the inside of his car’s sunroof. His design called for a 39″ (0.99-m) slot, so he laid out a U-shaped slot to fit the window and outlined it with copper foil tape. His method was a little complex; he applied the copper tape to a transparent transfer film first, then stuck the whole thing to the underside of the glass in one go. It didn’t quite go as planned, but as he learned in the duck tape antenna, the copper tape makes it easy to repair mistakes. A BNC connector with pigtails is attached across the slot about 4″ (10 cm) up from the end of one of the short legs of the slot; yes, this looks like a dead short, but such are the oddities of radio.

Is it a great antenna? By the numbers on [Ben]’s NanoVNA, not really. But any antenna that gets you heard is a good antenna, and this one was more than capable in that regard. We’ll have to keep this in mind for impromptu antennas and for those times when secrecy is a good idea.

41 thoughts on “A Quick And Stealthy Mobile Slot Antenna From Copper Tape

  1. This may be because many automotive glass applications have a metal oxide layer in the glass. This doesn’t work well with antennas.

    Most of the ones I’ve seen say “solar”… or similar on the label.

  2. If you make this antenns for your car remember any rf more than 10 wattd will create dangerous electromagnetic radiation for yhe people in thr car you need to be 3 metrr distsnce from antenna at mote thsn 20 watts unless you want to heat your brain g8wjy

      1. 10W EIRP is the threshold for doing an EMF assessment in the UK. Handhelds are well below that power when used with their antenna.

        The threshold and separation distances are based on ICNIRP exposure limits.

    1. Yes I did use a HT and it can push out 8W but at the same time that antenna is much further away from me than the rubber ducky normally on it. I do agree that once you get into above 20w it is worth a calculation but I still think it would be in safe levels up to about 50w

    2. 20W is completely safe. The first part of the spectrum with any danger is near the microwave segment. You can receive GHz RF that can heat tissue and liquids. But the dangerous radiation is from the sun. There is a reason skin cancer is a real problem, while radio cancer is not.

      It all has to do with the wavelength and the size of our DNA particles.
      Ionizing or non ionizing is the important difference.

      People without sun screen babbling about the dangers of 5G is i joke.

      1. That is awesome bro! Do you have a special way to fold your hat or do you just get a giant piece and crumple it to the shape of your Mellon? I like to fold mine so its like a stealth fighter almost so it bounces incoming radar signals at the same time! Lol

    1. Reminded me of Hogan’s Heroes tv show….Every time they needed to contact the underground the antenna extended up through the stalag flag pole. They hid that from Colonel Klink and maybe that would be a way to hide it from the HOA. ( I make no comparisons between HOAs and a past German dictatorship… That’s up to you. 😁)

      1. My WW2 radio truck is totally earthed everywhere. Inside the truck, there was hardly any rf, no matter how much power they transmitted or were parked next to. It was used to contact the underground also.

      2. I’ve yet to encounter an HOA that couldn’t be beaten senseless (figuratively) with it own rule book. The few HAM operators I’ve met got as much of a kick out of hiding their antennas from their HOAs as they did actually talking on their radio sets.

        Of course these are also the same guys that complain about LED Christmas lights blanking out certain bands and “leaky” microwave appliances, so I don’t know how much of their babble to believe when they corner me at holiday parties.

    2. LOL the next video I am working on is dubbed the ‘Flagi’ a Yagi on a flag. This is a good idea but slots and Yagi antennas are directional. One more so than the other so you would be at the will of the wind and or lack of wind if you do not support the flag horizontally. Anyhow a flag antenna is on my list of things to experiment with. THanks for watching and the feedback

      1. In Japan you see advertising flags everywhere. They are rectangular and 4:1 height:width proportions. Because it isn’t terribly wide, there an elbow at the top and a horizontal stick to hold the flag up even when there is no wind.
        In the US, you can get something similar, but with only a vertical pole that curved into a leaf shape. You could make your own with fiberglass (for rf transparency) easily enough.

    3. Federal law mandates that antenna for radio operators (professional or hobbyists) cannot be restricted. It may require a permit to put up, but HOAs cannot force you to take down or prevent you from installing an antenna. I live in an HOA and down the street is a 30 foot HAM radio antenna and someone in the HOA tried to get it taken down. He sued, won in court and the HOA ended up misappropriating funds and got turned into a sub-municipal district so instead of HOA dues it comes out of property taxes.

      Basically because of the whole thing the HOA is now powerless. Lol

  3. Just so you know: Some copper tape conducts through the sticky side, others do not. rom comments on salespoints I expect it’s less than 50% that does

    So test your tape and if it does not then fold a bit on the end to create a conducting edge.

    1. It is true that “Some Cu foiltape conducts on the sticky side and some do not.” I Iined the cavities of my electrical geetars with it and created entirely new universes of floating grounds!

    1. I’m now curious also. I would say you would want the base station situated along a wall with fair reception to the strongest cellular tower in your area. The alarm sensors should cause an error if they do not satisfy the base station communication protocol. If your building is metal you would need to put it in a window but maybe camouflage it in a flower pot (with fake plant also) to minimize it becoming a target.

  4. What’s the advantage in this case of using a slot antenna rather than for example a folded dipole? It seems to just use more tape and doesn’t actually provide any significant ‘plate’ so I don’t understand the point.

  5. I think IF there might be an a RF radiation problem, this could be easily fixed. Most sun roofs/moon roofs I’ve seen have a inner cover that you can pull shut to block out the view. Simply line the topside of it( the part that faces the sunroof window) with RF blocking material…. And when you are going to use your radio, pull the sun shield panel closed. This should block out any radiation emitted…

  6. Hello there gents, how about just hanging the Flag antenna concept, on a side of yer house, in the direction you communicate?
    Still sounds like neat, stealthy idea 💡!!!!

    Hogan !!!??????!!!!!!!!

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