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Field Day Antenna!
by
Paul - AE5JU
(Updated 09-28-2012 with
new mods info)
This past Field Day the club tried to
use a folded loop that just didn't work.
All contacts were made on my
portable antenna. The club asked me to make something for them that would
work on 75, 40, and 20 meters.
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, too.
Here it is:
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.
I made my calculations, cut my
wire to length, etc. Left a few feet on the ends so that it could be let
out.
The 4 legs are all about the same overall length. 40 meter
legs go one way, and the 75/20 meter legs are at right angles. The wires
double as the guys, with insulators out at the ends, and then 550
parachute cord going on out to tent stakes... or anything else you can
find to tie to.
The 75/20 legs have coils in the middle of each
leg. These are placed at about where the ends would be if the wires were
cut for 20 meters only... traps or chokes. They also act as loading coils
so that the overall length will work for 75 meters.
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 type.
"550 Parachute Cord" extending out to tent stakes.
The recipe:
Wire used is 14 ga stranded insulated wire, the 500' roll for $25
from Home Depot.
I had a lot of black parachute cord, but will buy
some more orange for the end rope on this. Also, my plastic tent stakes
are green. Difficult to see in the dark. I'm going to get some bright
yellow tent stakes.
The center insulator is cut from a cheap
plastic kitchen cutting board from Walmart. A half inch dia hole was
drilled in it, and a SO-239 socket screwed to it. The top of the socket
was sealed with Aleene's 7800 adhesive.
There is approximately 3"
of wire from the socket to each of the four holes, or tie points for the
wire legs.
The two 40 meters segments go one way, and the two
75/20 meters segments are perpendicular to the 40 m.
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
ends.
75 and 20 meters segments:
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.
Measurements for the inner 20 meters segments are
from the tie points on the center insulator out to the first turn on the
coils. The measurements for the outer wire segments are from the last turn
of the coil on out to the hole in the ceramic "dog bone" insulators on the
ends.
The coil formers are 2" ID PVC pipe, which is 2.375" OD. I
have holes drilled in the pipe to secure the ends of the coils with
ty-wraps. After tuning was deemed finished, the coils were wrapped with
Scotch 3M electrical tape. I find Scotch electrical tape does not turn
gummy and fall off with age. Good quality tape is worth the money. I made
the coil formers with "J-slots" on the ends, to allow more turns to be
taken off or added for tuning. Now that I have the final measurements that
will not be necessary when making future copies of this antenna.
Due to the diameter of the 14 ga insulated wire, I was able to get
right at 9 turns per inch, close wound, that is, turns touching each
other. I used this 9 turns per inch figure in an online coil design
calculator. This gave a nice repeatable build on the coils, and a length
to diameter ratio of about 1.75:1, which is right in the middle of the
suggested design ratios of 1.5:1 and 2:1.
Other
notes:
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.
Update!
New Additional Mods added
to antenna base below! 09-28-2012
Added 1" split rings (like for car keys) and snap
hooks
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 mods here!
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
Login to QRZ.COM for email address if you have
questions!
Feedback:
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 at http://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.
K5HDE
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