THE CHEAP QUAD ANTENNA AT WA6CDO|
San Francisco, California
This home-brew antenna cost just a few dollars, but gave excellent results during the year that it was up.
From November 23, 2000 when this quad was put into operation, through April 13,
2001 when this page was first posted, 776 contacts were logged on ten meters including
70 DXCC locations, WAC, and WAS, using an HTX-10, 25 watts peak on SSB.
The home-brew "carpenter's delight" 10-meter cubical quad against the northeast
The use of the center spider rather than a boom makes the antenna structure
a set of triangles, which while a bit more difficult to construct, is stronger
mechanically. The spreaders are 1" x 1"x 8' rough redwood, and cost about a buck each.
(The local Orchard Supply Hardware has bundles of bamboo, 6' long, for about $4 for 10 pieces which
would be ideal, except a bit short. Creative lengthening of the spider, or smaller
bamboo extensions on the end would be needed.)
Monofiliment fishing line is used to tie the reflector and
driven element together, making the entire box fairly rigid. Notice that the upper
and lower spreaders are of different length, and thus the angle at the spider
is different for top and bottom. This offset places the quad higher in
the sky relative to the rotor, while still obtaining the "stressed triangles"
for strength. I actually constructed these by forming the quad loops on the
garage floor, then used the full 8' length for the upper spreaders, and adjusted
lower spreaders until they met at the center, 3'3" above the floor. I trimmed them
there, measured the compound angles, and cut the parts for the spider accordingly.
The lower spreaders are a bit over 6' long. All wooden parts were given a coat of
Thomsom's Water Seal.
Another view of the quad against the western sky.
Dimensions are from the ARRL Handbook example: 35'7"
reflector, 34'7" driven element, spaced 6'6". The antenna is stranded 14 gauge
copperweld, attached to the ends of the spreaders with insulators made from
scrap plexiglass. Fishing line would probably do just as well. After mounting
the antenna on the rotor I fine-tuned the antenna to 28.5 MHz
with my MFJ 259 B SWR analyzer. I broke the reflector at the bottom and
temporarily inserted a coax connector, connected the MFJ 259 B, and
trimmed the reflector to 27.6 Mhz (3% lower). The coax connnector was removed
and the loop spliced to maintain that length. The driven element was likewise
trimmed to resonate at 28.5 Mhz. No other attempt was made to maximize gain
and/or F/B ratio.
View to the southwest, into some of the famous "little boxes" of San Francisco.
Perhaps this is why it is so difficult to work ZL from here?
The mast and rotor was originally installed for a television antenna. The
metal guy wires were about 8 feet long -- close to a quarter-wave on ten meters --
and contributed to my not being able to obtain an acceptable SWR.
I replaced the guys with polypropolene rope and everything fell nicely into place.
Closeup of the spider-hub.
Vertical mast is 2" x 2" fir, spider is 3/4"
pine. Assembly is with carpenter's glue and wood screws. Director loop and
driven-element loop are built separately in the workshop, then joined on the
mast with 1/4" threaded stainless steel rod, nuts and washers, on the rooftop.
One person can easily lift the entire quad up a stepladder and insert it into
the rotor mount.
Another closeup view of the spider hub.
The feed is an electrical quarter-wave section of 75 ohm coax coiled into a choke balun.
The feedline is 52 ohm RG 8X. This yielded a low SWR close to 1.1 at 28.4 mHz.
Two coax feedlines and the rotor cable pass through the roof
via this PVC pipe into a closet next to the operating position.
The operating position at WA6CDO as it was during 2000 - 2001.
The HTX-10 25 watt transceiver, the MFJ 971 antenna tuner with SWR bridge,
and the Astron RS-7A power supply are the stack in the middle of the bench. The transceiver has since been replaced by an Icom 718.
Contact me at: WA6CDO@jdboucher.com
WA6CDO Home Page
HTML coded and posted by jdb, 4/13/01, revised 9/14/02