开云体育

I'm wondering if someone can suggest a method, not involving using an antenna analyzer, to gradually construct this antenna so that it matches on all three bands?

I began by putting it together with the measurements suggested in the manual, and it didn't match at all on 20, and barely on 30.

Then I started again, tried a piece for just 20, got a match. Added a piece for thirty, and Trap A, and got a nice match on 30 - but no match on 20.

Help!

Hi Lloyd,
Generally, you start adjusting the highest band and work down through the other bands but as, you discovered, with a multiband trap antenna there are interactions among the bands so tuning is tricky. Since it's easier to trim than to add wire, on the first pass try for resonance at lower than optimum frequencies and make note of how tuning a lower band affects the previously tuned higher band.
Although this antenna is a great idea, there is a flaw in the tuner design that might make it difficult to achieve low SWR on all bands. The tuner uses an autotransformer so the shield of the coax feed line is actually the counterpoise. That means the 10' counterpoise isn't a counterpoise at all because the feed line is. What you get isn't actually an EFHW antenna but an off center fed dipole. That makes tuning on three bands even more difficult.
The best way to tune an EFHW antenna, IMHO, is with a tank circuit coupled to a feedline with a secondary winding that isolates the feedline from the antenna. See <> for an example. For the QRP Guys antenna you probably need to tune the antenna with a very short feed line, say, 10'. Once you have all three bands in the ballpark, try a longer feed line to see how that affects the tuning. You might need to try different lengths of feed line to make the antenna work on all bands. An alternative is to modify the tuner to accommodate a true transformer rather than an autotransformer.
An antenna analyzer certainly makes the job easier because it tells you where the point of resonance is. Without it, you're confined to making SWR checks within the band. Perhaps you can borrow an analyzer from someone nearby.
Hope this helps.
73,
Pat Byers? ? VE3EUR

you might be mistaking my antenna for a different one. this little tuner is connected to my radio via an 8" pigtail.

I borrowed an analyzer, a RigExpert AA-600.
hive-mind help needed.

how do you find the resonant length of a single wire, using a fancy gizmo? what I've tried so far is putting the wire in one side of a binding post-to-BNC connector, and no frequency anywhere near what I planned on had an SWR below the stratosphere. is there a different way to hook it up? so far, the demos I've found have people just screwing their feedlines' coax connector right on, no worries, but I'm trying to use this to find resonant lengths to assemble a multi-band EFHW antenna.

Pick a suitable range for the A-600, e.g. about 13~15 MHz for 20m and scan that range. You should see a dip in the graph pattern. Adjust the tuner for deepest dip (lowest SWR) which will be at the resonant frequency of the antenna. From that you'll be able to see if you need to lengthen or shorten the antenna. I would do this for all three bands just to see where you're at, then make the adjustments for the antenna segments starting at the highest frequency band. With the analyzer you'll also be able to see the interactions among the bands. Should be fun.
Hope this helps.
73,
Pat? ? VE3EUR

This is what I do:

Connect the Vector Network Analyzer between your wire and a counterpoise and
sweep the frequency of interest, looking for X to be 0 (neither negative or
positive). If the value of X is negative, your observed frequency is below
the resonant frequency, if X is positive you are above the resonance. X at
0 (zero) indicates your resonant frequency.

You will probably need a counterpoise (ground plane, a wire parallel to your
antenna wire) for your single wire to work against. For purposes of
measuring element resonance I have found the ground plane should be
physically longer than your single wire. A little longer is fine and a lot
longer makes no difference in my experience. Just make sure the AA-600 is
seeing the wire and not the ground plane. Easy to tell, just touch the wire
and the frequency of X at zero ohms will (should) move.

After you get the zero ohms X value to your desired frequency then look at
the R value to know what you have to do to get your SWR to match your
transmitter.

Works for me. Paul, W8AEF

I finally got a clue, and hooked this up to the EFHW multi-band tuner I'm building the antenna for. In tune mode, I now have between 1.2 and 1.5:1 on all three bands.

Lloyd K3ESE writes:

I finally got a clue, and hooked this up to the EFHW multi-band tuner I'm building the antenna for. In tune mode, I now have between 1.2 and 1.5:1 on all three bands.

Glad you got it tuned. Tuning a multi-band antenna always takes some work and piddling around. That SWR will get you out fine.

72, Paul NA5N

first trial, backyard, 12V supply to my ATS4A, EFHW sloping upward to about 25-30' at the far end.

Glad you got it all going.? I'm a bit late to this, but wanted to chime in as another reference point for others.? My initial thought was that you had the tuner in line with the first length working.? But, when you added the trap and 30m section 20m no longer worked.? That would be indicative of the trap not doing its job.
The resources at the SOTABeams pico trap page are very helpful.
Pico Traps Kit (pair)

|
|
|
| | |

|

|
|
| |
Pico Traps Kit (pair)

Find great deals on SOTABEAMS for Pico Traps Kit (pair). Shop HF and VHF Portable Antennas and Accessories for A...
|

|

|




In particular, check out page 5 of this document.? It shows how to check that your traps are at the frequency you want them to be:?
I often use the G0POT design 20/30/40m trapped EFHW when /P QRP.
72 de Bryan, k0emtBryan Nehl (@k0emt) ? Instagram photos and videos

Short form EFHW do not work well with traps easily.

To get 30M in the middle of 40 and 20 is difficult as 30M is not harmonically related.
To make it work you need a length that is a half wave for a multiple of a half wave at 40, 30,
and 20. 40 and 20 is easy. Its 30 that odd. with enough playing on paper you can get all three.

An easier way is a wire for 40/20 (about 66ft) and a replacement wire for 30 only or about
50ft.

However the problem is adding traps alter the wire length so you need the 30M trap
about 50 ft from the matching feed point. That will likely make the added length wrong
for 40M and 20M as it will behave like a load coil. ITs not the trap but its position and
loading of adjacent segments that is a problem to work out.

Keep in mind an EFHW is about 3000 ohms at the feed point at resonance and that
repeats every half wave. Traps work best at high current points or odd quarter wave
(middle of the half wave).

When I use mutiband EFHWs I stick to harmonically related bands or use wires cut
for a specific band. The latter give a more predictable pattern.

Allison

Another option is the way LNR's Trail-Friendly End-Fedz works. It
handles 40 and 20 because they are harmonically related. But there is
also what looks like a short coaxial stub partway down the wire, but
it's really just a shorting jumper that connects the two wire segments
together. (The cable stub makes it harder to lose.) You remove it to
use the antenna on 30m, effectively shortening the wire to the correct
length for that band.

A comment related to what Shirley said ... I have had good luck building a 3 band (40m/30m/20m) EFHW by borrowing and applying the idea of a
segmented dipole. The 40m EFHW is physically broken into 3 sections via two insulators inserted so that the first (when viewed
from the tuner end of the wire) isolates a 20m EFHW, the next section when added on via a jumper (I use alligator clips and spade lugs) is resonant on 30m
and addding the last section makes it an EFHW on 40m. In fact I discovered that the 40m EFHW can be used as a 20m end fed full wave so really you just need to isolate the 30m EFHW section with a small insulator.
I use an EFHW tuner from Pacific Antennas that is modified (ala AA5TB) to use a toroidal transformer rather than a tapped inductor and I just use the appropriate
length short counterpoise wire for each band. It works like a charm and doesn't require messing around with traps.

Cheers

Michael VE3WMB

--
Michael VE3WMB / VA2NB