This past Field Day the club tried to use a folded loop that
just didn't work.
Let's skip right to the conclusion. I used this antenna to check in with my favorite regional net on 75 meters with my Icom IC-718 operating at 100 w SSB from a RBC-6 sealed lead acid ("gel cell") battery. Net control and assistant net controls reported that I sounded good, one giving a signal report of "10 over" from nearly 400 miles away.
This antenna will work on 40 and 20 meters without tuner,
and with a little assist from a tuner will work on 17 and 15 meters,
This is a Field Day / Portable / Emergency antenna I just finished trimming.
An antenna analyzer made for easy trimming.
This is a multiband inverted V dipole. The club president
and I had discussed bands that needed to be covered, so it was decided
that 20 meters, 40 meters, and 75 meters were most important. Anything
else is extra.
When dipoles are this low (about 22'), proximity to the ground lowers the antenna's impedance, which causes havoc with SWR. To counter this, like the Buddipole, the feedpoint is just off center, enough to bring up the impedance a bit closer to 50 ohms. The legs connected to the "hot side" of the coax are a little longer, and the coil on the 75/20 meters leg has a few more turns on the coil than calculated. The legs connected to the "shield side" of the coax are a little shorter, and the coil on that shield side 75/20 meters leg has a few fewer turns than calculated. This off center feed allowed for a minimum SWR of 1.3 to 1.4:1, which could not have been achieved otherwise.
Let's see if this theory works out...
20 meters resonated too high, needed lengthening.
Due to the way this antenna is made it was decided to lengthen the 20 meters segments first by unwinding a turn from the inner side of one coil. The coil former, 2" pvc pipe, has J-slots cut in each end so that wire may be let out or taken in.
Letting out one turn, lengthening that side by about 7", brought resonance down some, but not enough. Letting out one more turn on that same side resulted in these figures:
14.000 mhz 1.6
14.100 mhz 1.5
14.200 mhz 1.4
14.300 mhz 1.5
14.350 mhz 1.5
Next I tackled the 75 meters segments, which were tuned by cutting off the ends of the wire on these same segments. I took off about 2' on one end, and about 18" on the other to give:
3.900 mhz 2.3
3.910 mhz 1.7
3.920 mhz 1.4
3.925 mhz 1.4
3.930 mhz 1.5
3.935 mhz 1.6
3.943 mhz 2.0
There are some emergency nets on 3.925 - 3.935 mhz.
20 meters was rechecked, and remained unchanged.
40 meters was adjusted by removing about 10" from each end to give:
7.000 mhz 2.9
7.100 mhz 2.3
7.150 mhz 1.8
7.200 mhz 1.5
7.250 mhz 1.4
7.300 mhz 1.5
After these changes all three bands were checked again to make sure nothing else needed to be fine tuned.
15 meters was found to be in the 3.5:1 to 3.0:1 range. A tuner can take care of that quite easily.
Loading coil. These coils are close wound on 2" ID pvc with 14 ga insulated wire. There are J slots on each end to allow adding or subtracting turns of wire.
Once the coil tuning was settled it was wrapped with 3M Electrical Tape.
Center insulator made from cheap Wallyworld kitchen cutting board.
100' of RG-8X coax was used, which is approximately 1/2 wavelength at 80 meters. Also, there are five Palomar Engineers FSB-1/4 ferrite snap on beads on the coax up by the feedpoint. These act as a "choke balun".
Base, PA Speaker tripod. The mast is 5 sections of fiberglass pole sections. These are military surplus camo net poles, an Ebay item.
End insulators shown above are ordinary ceramic dogbone
40 meters segments:
Measurements are from the holes, or tie points on the center
insulator out to the hole in the ceramic "dog bone" insulators on the
This 75 / 20 meters section is wired parallel to the 40
meters section, only these legs are stretched out at right angles to the
40 meters legs.
There are five Palomar Engineers FSB-1/4 ferrite snap on beads placed on the RG-8X coax near the feedpoint. This forms a "choke balun" to stop RF on the shield. Similar snap on beads may be purchased from Ham City listed with the coax.
The apex of the antenna is at approximately 22'. This includes the tripod base and 5 fiberglass mast pieces. These are the common military surplus fiberglass camo net poles sold at hamfests and on eBay for use as antenna masts.
The only drawback of the coils is that the 75 meters bandwidth is narrower than if the antenna were full length. However, the antenna still covers the desired portion of the band with good SWR, and a little more using the tuner.
* (see note at bottom of this post)
But the use of coil loading and the overall length of this 75 meter antenna (appx 50% of full size) causes no noticeable drop in signal. I still got good signal reports from others I regularly talk to, so they know what I usually sound like with my full size dipole.
While this is not a permanent antenna for me, this might possibly help others fit 75 or 80 meters into a small yard.
One added note: While many say coax length does not matter, during initial testing with an analyzer I was getting good, consistent readings on 40 and 20 meters, but the 75 meters SWR readings were squirrely. Then it dawned on me... 50' of coax. Considering velocity factor, that is very close to 1/4 wavelength at 80 meters. The next time I tested it I used 100' of coax, and that works out to approximately 1/2 wavelength at 80 meters. SWR readings were consistent and stable.
Lesson learned: The old "conventional wisdom" to use 1/2 wavelength of coax got to be "conventional wisdom" for a good reason. I related this to one of our old club gurus and he just smiled and said, "Toljaso." Length does matter, so use 93' - 100' of coax with this antenna.
* So, a thought occurred to me... What if you do want to go lower in the 75 meter band? Why not put some sort of connector near the ends, down by the insulators? Then clip on some added pieces of wire, 1' long each, or 2' long each, and string them on past the insulator, perhaps securing them to the parachute cord end ropes with tie wraps? That would drop the resonant frequency to a lower portion of that band.
New Mod to Field Day Antenna done with Anderson Power Pole connectors
Measurements of wire lengths are now from the Powerpoles to the coils or end insulators.
Note that you may also download a pdf file of the antenna construction with modshere!
Errecting the Field Day / Emergency / Portable Antenna
Slip round insulator over top tenon of the top fiberglass mast piece.
Attach coax. Use 75' - 100' of RG-8X. Do not use 50', or it won't work on 75 m.
Clip the four wire elements to the appropriate rings on the center insulator. Observe colors and stripes on the connectors. The two plain wires will be opposite each other, with red to center, black to shield. The two coil loaded wires will be opposite each other with red-yellow to center, black-yellow to shield.
Spread out the wires fully, then bring in the ends of the ropes about 10'--12' and stake to ground. I suggest bright yellow tent stakes to help prevent tripping hazard.
Raise center mast vertically, placing new sections in one at a time from the bottom. Use a total of 5 sections of mast. Place on tripod.
Now go around and adjust stake position, wire/rope tension.
You may want to add ribbons of Caution Tape to prevent clothes-lining.
Will work 75 meters (resonant about 3.920 mhz), 40 m, 20 m, with little to no touch up from the tuner. Will work 17 m, 15 m, and 10 m with light touch up from tuner. Have not tried it on 6 m, but suspect it might work there, too. Let me know.
We have worked, from the Louisiana Gulf Coast, from coast to coast on Field Day, NY to CA, on 20 m and 15 m. Worked all over the Gulf Coast states, FL to TX on 40 m. At night I've done a regional net, LA, MS, TX, AR, OK, on 75 m at 100 w running off batteries, with good signal reports.
75 meters, due to the coil loading, is somewhat narrow. A tuner helps.
There you have it...Enjoy and have fun! AE5JU - Paul
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Just wanted to pass a note along.
Had the opportunity to work the digital side of station K5R - Katrina / Rita 5th Anniversary. Fun and rewarding. To the point I had to bring all the gear I was going to use to the SELARC club station. I decided to build a copy of your field day antenna as described athttp://www.hamuniverse.com/ae5jufielddayantenna.html
It worked like a champ. I tuned it in the backyard and hoped it would work on the roof of the Ponchatoula Community Center. Once set up on the roof I checked the SWR with a MFJ antenna analyser. 20m was spot on. 40m was a bit off at 2.2 but good enough. Not worth the hassle of going back up there to tweak it. The tuner handled 30m so I could use that also.
Ended up with 60 QSOs from 28 states and 3 DX entities. Thanks for the antenna design.
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