Portable 28 MHz (10 Meter) 2 Element Beam
Using
102" Steel Whips
by
KL7JR/VE8RST/VY1RST
The old standby 102 inch steel CB whip has
been around for a long time and used on the amateur bands by many hams
over the years. This design will incorporate four of the steel whips and
allow us to concentrate our signal to a specific area and give us 3 to 4
dBd to boot (*)! It is direct feed with 50 ohm
coax. It will also be easy to assemble and fairly transportable at
a cost of about $100 depending on how you connect the whips to the boom or
use new or used materials. Here's a method I chose which will make it a
snap to assemble and install in cold weather, field day,
etc.
Version above is for 28.450mhz
A 102 inch
steel whip is a quarter-wave on the CB band (27 MHz). Using the N3DNO antenna calculator we arrive at the following
dimensions for our desired frequency of 28.450 MHz (+-): driven element is
8 ft and 4 inches long each (there's 2 for a total of 16 feet 8 inches),
the reflector is 17 ft and 7.3 inches long with the spacing at 6 ft and 7
inches from the driven element. Note that
the reflector is one continious piece electrically. (Not insulated
from the boom.)
Now, keeping that in mind, and trying to
maintain the 8 ft and 6 inch length of the CB whips (I didn't want to cut
the whips down as I may want to reuse them some day for other projects) we
find these dimensions (trial and error using the above antenna
calculator): for 27.8 MHz the driven element is 8 ft and 6 inches long
(the actual length of each whip+-) on each side, and the reflector is 18
feet long (**) with a spacing of 6 ft and 9 inches. The difference in the
desired frequency of 28.450 MHz vs. the "not to cut the whips" frequency
of 27.8 MHz is only .650 MHz which should be easily handled by any antenna
tuner.
To mount the whips to the boom, you can
u-bolt a short piece of "L" channel (+- 6 inches long with the predrilled
holes) to the boom and then mount the antennas to that using insulated
connectors for the driven elements and non-insulated connectors for the
reflector.
(Or if you are like me and want something a bit more
attractive, you can use a pair of
commercially made mounts (MFJ mini
dipole mount #347 or others like in the picture
above).
The 102 inch steel whips are available from DX Engineering
(#DXE-WP-102) or
through Radio Shack (?). This
design may also work on 12 meters as well. This antenna is in the
experimental stages, but the above dimensions should be
good starting points, and you may chose to cut your whips down to the
design dimensions above.
Keep in mind that whether or not you use the
commercial mounts on the reflector, or your
own mechanical design, that you will have to make certain both
whips are connected together forming one continious electrical connection.
Other wise it will not work! If you have trouble tuning for lowest swr or
just don't want to bother, then use your tuner. A 1:1 balun at the feed
point is also suggested.
Any and all feedback is appreciated. 73 de
Yukon John, KL7JR
kl7jr at
Yahoo.com
*See "2 Element Wire Beam for 28 and 24
MHz" on this website
** You can add 6 inches of wire to each of
the 102 inch whips to get the 18 ft length, or not. Experiment either way.
Recent Comments from builders!
"I was bored
yesterday, and was surfing the net for an antenna I could build with what
I happen to have around the shop here...
I had to gain the height by
placing the antenna mast onto my motorcycle trailer and then run it from
my 2000 Mercury Sable LS, using an Yaesu FT-817ND with a LDG Z11Pro
Autotuner and about 50ft of RG-8X that mostly came from Radio
Shack..."
"About 14:53 Hours EST, I made QRP contacts with KJ9B in
Honduras, and later made contact with PU5OGE Evan in Corocheba Southern
Brazil on 5 watts."
"Wow, I built the antenna in about an
hour, with parts that I had just laying around drawing dust, and was able
to make these contacts....the Skip was great, and even heard Argentina,
but was unable to confirm contact...Thanks for such a great Idea...
I will be building my next antenna from the formulas from this project
below:
2 Element Wire Beam for 28 and
24 MHz
http://www.hamuniverse.com/kl7jr1012wirebeam.html
Thanks,
Robert
KJ4OTZ
Graham, North Carolina
27253
