Little antenna "experiment"

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Andrew (grayhat)
Posts: 125
Joined: Sat Jun 11, 2022 2:59 pm

Little antenna "experiment"

Post by Andrew (grayhat) »

Being curious, as I am :D and since the idea has been whirling in my head for some time now, I decided to run a little "experiment" regarding the (not so) "random" antennas, for such a purpose I created a model in 4NEC2 describing a somewhat typical "portable random" setup, in short the starting point was a random antenna with a 41ft radiator and a 17ft counterpoise, in the the model, the antenna feedpoint is at 5ft from ground, the radiator is sloping up with an endpoint at 20ft (which I considered reasonable for portable operations) and the counterpoise is just dropping down to ground and is then laid in the same direction as the radiator, now, the idea was to try and find if a particular combo of counterpoise length and transformation ratio could achieve a good SWR over the 7 to 30 MHz range and below you'll see my findings

In short the image below shows three NEC "scans" for SWR over the 7 to 30 range, the top one shows the original antenna fed using a 9:1 UnUn, as you can see, while the SWR is below 10:1 it isn't exactly optimal, so I started raising the counterpoise length until I reached the same length of the radiator, and the middle part of the image shows the curve of the same antenna but using a 41ft (instead of a 17ft) counterpoise, it's easy to see that things seem to have improved a bit, but I wasn't still satisfied, so I decided to replace the 9:1 UnUn with a 16:1 one (800 to 50) and the bottom part of the image shows the results, the max SWR is 6:1 at 12 MHz, but it stays at or below 2:1 on most frequencies and the second peak, at 4:1 is around 25MHz, so overall it seems to me it may be considered a good match, not only the mismatch with the coax feeded will be reduced, but even a "poor" antenna matching unit should be able to easily find a match

Nothing special, but I hope it may be useful (note: right click and open image in new tab to see it a full res)
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KnoxvilleMike
Posts: 9
Joined: Fri May 27, 2022 8:11 pm

Re: Little antenna "experiment"

Post by KnoxvilleMike »

Nice work. With regards to both EFRW & EFHW antennas, does raising the feedpoint/transformer height above ground make a difference in impedance or efficiency? I'm planning on installing a 132ft EFHW at home, hoping to get the midpoint & far end at 30+ feet. I've been wondering if it will make a difference if the transformer end is 10 or 20 feet off the ground.
Andrew (grayhat)
Posts: 125
Joined: Sat Jun 11, 2022 2:59 pm

Re: Little antenna "experiment"

Post by Andrew (grayhat) »

KnoxvilleMike wrote: Tue Aug 08, 2023 1:13 pm Nice work. With regards to both EFRW & EFHW antennas, does raising the feedpoint/transformer height above ground make a difference in impedance or efficiency? I'm planning on installing a 132ft EFHW at home, hoping to get the midpoint & far end at 30+ feet. I've been wondering if it will make a difference if the transformer end is 10 or 20 feet off the ground.
Didn't play with moving the feedpoint or the endpoint, mine was just a quick and pretty "raw" NEC simulation to see if I could somewhat improve the matching, as for your antenna... either a 119ft or a 148ft "random" fed using a 9:1 and with a good counterpoise would allow you to cover all bands from 10 to 160 meters, as for difference, for sure (and from previous NEC models and direct experience) the antenna will be a bit more efficient since ground losses will be reduced, plus the launch angles will be a bit lower (depending on bands, by the way)

If you can describe your antenna (radiator length, counterpoise length, radiator feedpoint height and endpoint height, counterpoise layout and heights) it won't take much to me to put together a NEC model and see what comes out of it
Andrew (grayhat)
Posts: 125
Joined: Sat Jun 11, 2022 2:59 pm

Re: Little antenna "experiment"

Post by Andrew (grayhat) »

KnoxvilleMike wrote: Tue Aug 08, 2023 1:13 pm Nice work. With regards to both EFRW & EFHW antennas, does raising the feedpoint/transformer height above ground make a difference in impedance or efficiency? I'm planning on installing a 132ft EFHW at home, hoping to get the midpoint & far end at 30+ feet. I've been wondering if it will make a difference if the transformer end is 10 or 20 feet off the ground.
Just a note (and a bit OT but... WTH :D)

Since the antenna you described above is for "home" use, did you consider putting up an alternative to the EFHW ? I am referring to the G7FEK antenna which is described here

https://www.g7fek.co.uk/blogus/newsshow ... nten_29507

https://rsars.files.wordpress.com/2013/ ... s-1-31.pdf

not only it doesn't need to be very high, but it also offers low takeoff angles and allows covering six bands, including the 80m one, so I think it may be worth considering it before starting your antenna installation project :D

Oh and also check this

https://www.eham.net/reviews/view-product?id=11056
KL7MJ
Posts: 49
Joined: Mon May 15, 2023 9:06 pm

Re: Little antenna "experiment"

Post by KL7MJ »

Nice work Andrew!
I'm beginning to think that an EFRW is a great general purpose antenna, so long as you have a tuner. I have a 35.5/17 EFRW that does very well for me, but I think that the longer the antenna and radiator, the better.

My current antenna project is a travel EFRW using a pair of slinkies, with a 9:1 balun. It may not be great, but I hoping it will be good for use in a hotel room.

73,
Mike, KL7MJ
Andrew (grayhat)
Posts: 125
Joined: Sat Jun 11, 2022 2:59 pm

Re: Little antenna "experiment"

Post by Andrew (grayhat) »

KL7MJ wrote: Fri Sep 08, 2023 12:30 am Nice work Andrew!
I'm beginning to think that an EFRW is a great general purpose antenna, so long as you have a tuner. I have a 35.5/17 EFRW that does very well for me, but I think that the longer the antenna and radiator, the better.

My current antenna project is a travel EFRW using a pair of slinkies, with a 9:1 balun. It may not be great, but I hoping it will be good for use in a hotel room.

73,
Mike, KL7MJ
Hi Mike, thank you !

As for the EFRW, its main advantages imHo are the ease of installation, the coverage of all bands, the fact that even if shorter than a dipole/doublet it may still be used on lower frequencies (with reduced efficiency btw) and then, it's almost "stealth"; on the other hand the EFRW is, as many other antennas a compromise, so we should accept some limitation, but I think we may live with that :)

As for the tuner,, willing to spare some money, one may just build it, in such a case I suggest to look at this one

https://pa0fri.home.xs4all.nl/ATU/FRIma ... tcheng.htm

and in particular, at the schematic of the DL4CS version found further down the page, it doesn't go down to 160m but since it doesn't need a "band switch" it's easyy to build, compact and can be easily turned into a remote ATU just by using a pair of stepper motors :) in such a case, the EFRW could be fed directly from the ATU (installed at the antenna) w/o using a 9:1 ;)

As for the length, longer isn't always better, set aside the losses due to wire resistance, we should consider that, at higher frequencies, where the radiator size may be several wavelengths, the antenna will become more and more directional toward its endpoint (which may be desirable or not), plus a very long antenna will present mechanical issues making the setup more complex, my usual "go to" to pick an EFRW lenght is this one

https://udel.edu/~mm/ham/randomWire/

and I believe that 203ft is the limit for an EFRW, and then we should consider how we'll be going to install it, a good way (example with a 137ft radiator) is placing the feedpoint at (say) 6ft from ground, going up to about 33ft (or more) and from there sloping down to about 17ft, the resulting "inverted L" setup will give pretty good radiation pattern

As for the "slinky", that's made of steel, so its radiation efficiency will be much lower than copper, not that it won't work, but we'd probaly loose quite some of our precious W as heath :(
Last edited by Andrew (grayhat) on Sat Sep 09, 2023 5:45 am, edited 1 time in total.
KL7MJ
Posts: 49
Joined: Mon May 15, 2023 9:06 pm

Re: Little antenna "experiment"

Post by KL7MJ »

Thanks Andrew - that website has a lot of good info!
I bought one of those cheap (bad) QRP antenna kits online, but plan to use the parts to make a z match instead of a t match. I like George Dobbs design in his book QRP Basics. It's a pretty standard design - similar to several of the designs on that site, and I like that it doesn't have a variable/tapped coil.
I know that slinkies are far from the best conductors out there, but I'm looking forward to testing the slinky EFRW against my homebrew magloop!
Andrew (grayhat)
Posts: 125
Joined: Sat Jun 11, 2022 2:59 pm

Re: Little antenna "experiment"

Post by Andrew (grayhat) »

KL7MJ wrote: Sat Sep 09, 2023 1:45 am I'm looking forward to testing the slinky EFRW against my homebrew magloop!
why so, use an antenna for TX and another one for RX, as far as I know, there's no law forbidding it :D
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