Trapping the EFHW?

[KL7MJ]

My primary antenna is a 66-ish foot long EFHW. It covers 10, 15, 20 and 40 meters. I would like to add the ability to operate on 17m and 30m without a tuner.

I've looked at several 20/30/40 trapped EFHW designs, and they are all roughly 66 feet long and all have a 20m trap as well as a 30m trap. I would think that it should only require a 30m trap, because the full length should be resonant on over multiples of 7MHz (to include 14MHz).

Does a trap prevent an EFHW from radiating on each multiple?

My thought is that with a 17m and a 30m trap, a 66-ish foot EFHW should work on 10/15/17/20/30/40 meters - or am I missing something?

73,
Mike, KL7MJ

[SkipRD]

So, I may be about to display my ignorance, but I think if you want to insert a trap to work on 30m, you'll also need one for 20m. The reason is that a 66' antenna with a 30m trap will look like a 30m antenna to a 20m signal and it won't be happy. In other words, the 30m trap will look like a half-wave (i.e., 50-ft'ish) length of wire, which isn't resonant on 20m.

One other thought, I also wanted 30m on my 40m EFHW, but I just installed a link at 50'feet'ish. It's not quite as convenient, but I can usually lower the antenna slightly, pull the link, and re-raise the antenna in just a couple minutes. And I guess it's a little more efficient than using a trap.

Curious if others chime in and give a better explanation.
73 Skip K4EAK

[Andrew (grayhat)]

Here's a "crazy" idea, use a tape measure as your antenna

You connect your feedline (ok the transformer) to the end of the tape, extend the tape to the desired length lock the measure to that and hang up the spool then you may also run some tests "at home" to find out the lengths and use a permanent marker to have a quick reference for the various bands, while the radiation efficiency won't be as good as the one from a run of copper wire, the antenna should work and when packed up (ok, wrapped) will take very little space in your portable kit bag

[kc0myw]

Here's a "crazy" idea, use a tape measure as your antenna

I think that's been done before with some success. I think there was even something about a modified tape measure specifically designed with this in mind.

However, and KL7MJ should correct me if I'm wrong, I believe the idea with the traps is to be able to have it be resonant on the mentioned bands without having to lower/adjust/raise the antenna on each band change.

[KL7MJ]

That's exactly what I'm looking for. My goal is that I can band switch at my rig without having to drop the antenna. I've been trying a 35/17 random wire with an antenna tuner, but my FT817 goes crazy (switches bands/frequencies and refuses to transmit for more than a second or so) on 15m and 17m, so I'm looking for a easy to use solution that includes those bands.

The question really is if a 40m EFHW (66 feet +/-) with a 17m trap can still resonate on 10/15/20, or is it now limited to 17/40? I plan to test it sometime soon, but if it is already established fact I'll design an antenna with more traps.

Thanks.

73,
Mike, KL7MJ/4

[N8TGQ]

Hi Mike

I went thru a ton of stuff online and in my stash of info here, and I can't find anything about using a trap and still having the multiple-halfwave advantage of basic EFHW.
Even for a basic 40-30-20m using a trap for 30 meters also had a trap for 20m.
I'm not saying it's impossible, but if it is, why is there no info anywhere? Best I could find was using links at 30 and 17m.

We were talking about remote control links on another forum. With little bluetooth controlled arduinos, mini latching relays and a tiny battery from one if those mini drones, you could build a r/c link smaller and lighter than what trsps were just a few years ago! Charge them up when you charge your radio. Then control them from your phone! If you market this idea, I get a cut!

I'm using a random wire here at my apartment. Same length as you, but the counterpoise is around 25' and it all runs direct to an MFJ 971 tuner. It tunes well on everything 40m and uo. Maybe experiment a bit with your wire lengths. Are you using any kind of choke on your feedline?

Remember it's all fun!

Rick N8TGQ

[KL7MJ]

So I decided to build a trapped EFHW to find out...

My design was a 4-band EFHW - 17/20/30/40. I used three traps from SOTABeams.co.uk (17, 20 and 30m traps). My first design used wire cut for a half wavelength for each band:
17m - 26 feet
17m trap
20m - 7 additional feet
20m trap
30m - 9.5 additional feet
30m trap
40m - 24.5 additional feet
and this was what that gave the first plot below.

Initial antenna

4befhw_1.png (11.56 KiB) Viewed 6492 times

I spent the rest of the day tweaking the settings, and I learned several things:

Traps lower the resonance of the antenna, so you have to shorten the wires.
The 20m trap was necessary (I removed it and quickly replaced it - the final "natural" resonance was around 17MHz).
You set the resonant point for each band by adjusting the trap, and you adjust the SWR by adjusting the length of the wire.

When all was said and done, the second plot below shows the final antenna. Once everything was tuned, I coated all of the splices and traps with liquid electrical tape - and rechecked the SWR and didn't see any change.

Final antenna

4befhw_final.png (14.82 KiB) Viewed 6492 times

The one thing I would do different is that I would have ordered a bunch more trap kits - I only ordered two, but I expect that I will use them a lot in the future. The kits are inexpensive but shipping isn't!

73,
Mike, KL7MJ/4

[N8TGQ]

So you did lose the resonance on 15 and 10 meters.
Thanks for going thru all the work to find this out, Mike. Your efforts are appreciated and I learned something!

[KL7MJ]

Yep, the shallow dip around 16.5MHz is the untrapped 20m dip.

[NJ6FNJ6F]

Greetings!
Out of the box suggestion - How about an 8 foot Delta kite. I use one with a 5 knot+ wind, and will take my resonant vertical dipoles for 40 or 20 up 200 feet. Just clip onto center of kite top radiating part of dipole and lower half just put a colorful streamer on it to have wind take it away from the small RG174 coax with ferrite core clipped on end of coax. You don't need an end fed antenna if the kite takes it up and your using light weight coax!
You don't have to fight with tree branches. I have 3-100 foot sections of very small light weight RG174 coax with coupling connectors which you can tape up to reduce stress, that takes up to 600 watts on HF...if they don't hear you the first time hi... So you have a hand winder for the string at a 45 deg angle to kite with an metal spike anchor in the ground, then have a hand winder that holds 200 feet of RG-174 coax straight up from the station which you let out as it goes up which only takes 1 minute. I adjusted my dipoles to freq numerous times up and down just pulling on the coax because it went so fast. I use the other 100 feet to run to the cars PL259 mount utilizing the cars chassis ground and I wind a few turns of coax around one of my VHF antennas on the car trunk so I can tell if kite is going down or up for reference. I sit in the air conditioned Camry with my FT-100-D Yaesu. On 20 meters at 2PM on the bay, I got a 5/8 in Milano Italy using 100 watts on USB the from San Diego bay. So it does work and gives you a 200 foot elevation to boot away from the power lines. 8 foot Delta kites are very light weight and stable, takes seconds to put together out of a back pack and all you need is a little wind which many places have too much of. So you can clip on whatever dipole you need and be done with it and suffer NO trap loss with an efficient resonant vertical dipole antenna. You just keep the kite above the car in a relatively fixed location with this stable kite and just pull the coax to bring the kite down for another band change. Having you kids fly the kite will give them something to do as well hi. You might even get more traffic when they hear you are kite portable... Works for VHF/UHF as well if your in a valley and need to get to a repeater. 73 from NJ6F / Rich