My first magic carpet ride

[KL7MJ]

I've seen several videos lately extolling the virtues of a faraday cloth, aka magic carpet as a replacement for radials. So for about $10 I bought a 44x108" faraday sheet.
I tried several configurations of a short base loaded vertical (6' antenna and base coil on a small tripod). I tried the antenna sitting on the cloth, connected by a jumper, connected by a jumper with a tuned radial, and finally with just the tuned radial. In short, without a connection to the cloth had high SWR, connecting it with a jumper lowered it to about 3:1, adding the tuned radial reduced it to about 2.5:1, and removing the cloth and just using the radial lowered it to about 1.3:1.

In this graph, the antenna with the jumpered cloth is the green trace, and the antenna with the tuned radial is the blue trace

comparison.png (13.81 KiB) Viewed 6856 times

Has anyone else tried one of these? If so, how were your results?

73,
Mike, KL7MJ

[John VA3KOT]

I haven't tried a Faraday cloth but I have experimented with many different kinds of counterpoise/radials. I found there is interaction between the lenghts of the whip and the radials. Did you re-adjust the loading coil or whip length after connecting to the Faraday cloth? In one of my experiments I stretched the vertical element way beyond its usual resonant length and used very short raised radials to bring it back to resonance - it works. The length of radials laid on the ground also affects tuning, although not to the same degree as raised radials.

[KL7MJ]

So a little more detail on my experiment.
I had the antenna sitting on my deck, which is about a foot off the ground.
The radial is about 16 feet long, and I laid it along the deck and onto the (clay) ground. About 10 feet was on the ground.
The first test I did was to place the antenna on the screen without a connection. It showed SWR inflections around 13MHz and 18MHz. Once I connected the ground screen or the radial the pattern was for a single inflection in the 20m band.
I didn't try to adjust the antenna for the ground screen in this chart, but adjusting the length of the antenna whip only moved the frequency up and down, it did not improve the SWR.

I designed the antenna to be resonant on 20m. If I have to significantly change the dimensions to make it work with the ground cloth I'd imagine I'd possibly get a lower SWR but at the cost of efficiency (which already isn't great in a 6 foot tall antenna).

[N8TGQ]

Great writeup, Mike! I appreciate your time and efforts.
I wonder if adding a few more jumpers to the faraday cloth at different points would change anything? Maybe the fabric isn't very conductive.

I'm just guessing! I have absolutely no experience with this stuff.

Rick N8TGQ

[KL7MJ]

Rick,
Ok, I did a little more testing that I left out...
Adding a second jumper raised the resonant frequency but did not improve the SWR. Using both the cloth and a radial was better than the cloth alone, but worse than the radial alone.
I'm working on a homebrew PAC-12 antenna, and I'm hoping that a little bit bigger antenna might work better. As it stands now, I'm not overly impressed with the magic carpet. Glad I only paid about $8 for it!

73,
Mike, KL7MJ

[Andrew (grayhat)]

I try to stay away from anything "magic" or based on something which (apparently) violates the know physics laws, that being said, the longer size for a "normal" vertical antenna is around 6.5m (about 21ft), that is about 5/8 Lambda on the 10 meters band, the choice isn't random, see... the takeoff angle of a vertical will raise toward NVIS when it's more than 5/8 wave, so a 21ft vertical with an adjustable loading coil will offer pretty good radiation angle at 10m and, inserting some coil turns, will give you a match on the other bands, although its efficiency will probably be acceptable only going down to 20 meters

As for ground, either some radials and/or some chicken wire fence (metal) will give you a good ground system and both help lowering the antenna takeoff angle and ensuring your RF isn't just loast in the ground, and then there are several ways to "load" an antenna, including the so-called "capacitive hats" and the linear loading, but I think it will offtrack the discussion

Bottom line, try a 21ft vertical radiator with a tapped loading coil (to cover multiple bands) and some counterpoise wires, I think it may "float your boat", as for the counterpoise wires, sizing those at 1/16 or 1/8 Lambda and using 2 of them for each band ma be fine, then you may reduce the counterpoise wires count by size, just remove the lower frequency ones which (almost) match the counterpoise(s) for some higher frequency band

[KL7MJ]

I've been trying to pout together a kit for a trip to KL7-land that won't take up a lot of space/weight in my suitcase. I was hoping that the cloth would be a good replacement for a bunch of radials, but I'm not sold.

I built a PAC-12 antenna this weekend. It is small and light, and tunes up pretty well after a little tweaking of the length of the coil. On 20m I'm getting around 1.5:1 with four 15 foot radials. My next goal is to make an 80m coil for it that will allow me to check in to the Alaska Motley Net while I'm there.

I think I have a longer antenna whip somewhere in the garage - I'll have to try using it now that the weather here is getting better for outdoor experimentation!

[N8TGQ]

Good luck with your experiments. Playing with antennas is one of my favorite of the hobby!

I never had much luck with portable verticals. I could get them working well, but never could get the same result the next time I put it up. They worked great in a permanent setup, though.

[KL7MJ]

I think I enjoy making antennas more that I enjoy building radios (maybe not, but there are a lot cheaper).

Repeatability is the goal for this little antenna. After changing bands and adjusting it a couple of times yesterday I glued down the coil to keep it from changing in value. Tonight I'll see if that affected the tuning at all.

[KL7MJ]

I tried the PAC-12 again last night. 20m was right on, so I moved to 40m. Adjusting the loading coil I was able to get a dip in the SWR at 7.05, butit was about 2.4:1. I tried connecting the magic carpet again, but it really doesn't play well with the antenna. I guess I need to add some more radials to get the SWR down (my goal is 1.5:1 - 2:1).