Originally, this antenna was dropped off
at a local agency that sells donated
items. For some odd reason, they
threw away all the other donated
antenna parts. Fortunately, this
antenna had 18 foot long parts, and I
was able to get it 'to scrap it for the
aluminum' which saved them the trouble
to break it up so they could throw it
away. Actually, this is a story all it's own.
The original dimensions tuned out in
the 10 meter ham band. It consisted of
4 elements 14 foot long, 3/4 inch in
diameter, with 5/8 inch diameter tips to
get the required lengths. The boom
was originally 12 foot in length.
The parts are heavier than any design
I've run across before, and the original
manufacturer is unknown.
at a local agency that sells donated
items. For some odd reason, they
threw away all the other donated
antenna parts. Fortunately, this
antenna had 18 foot long parts, and I
was able to get it 'to scrap it for the
aluminum' which saved them the trouble
to break it up so they could throw it
away. Actually, this is a story all it's own.
The original dimensions tuned out in
the 10 meter ham band. It consisted of
4 elements 14 foot long, 3/4 inch in
diameter, with 5/8 inch diameter tips to
get the required lengths. The boom
was originally 12 foot in length.
The parts are heavier than any design
I've run across before, and the original
manufacturer is unknown.
My design is based on an antenna I found here: http://www.signalengineering.com/ultimate/4_element_yagi.html
(Thanks Scott!) This antenna is designed for 27.555. I standby on channel 19 (27.185 MHz) and redesigned the
dimensions for this frequency. The graph above shows the predicted performance. 14dBi of forward gain should
roughly be equivalent to increasing power off my vertical from 4 to about 60 watts. The F/R ratio of 29dB should be
the equivalent of a 1kW station off the front of the beam, dropping to 1 watt when off the back. The F/B of 41.5 dB
(180 degrees or straight off the back) should be a difference of 8 thousand times. I think I got the numbers right. In
any case, according to the predictions, it should be more than satisfactory. Unfortunately, the Yagi for Windows
program only works for horizontal antenna modeling. Performance will be less in the vertical mode, I assume due to
interference from the mast being in line with the elements. If anyone has time to play with my numbers with an NEC
program for vertical mode, I'd love to hear from you. Email I use the radio for mostly local communications and
will be vertical 90% of the time.
(Thanks Scott!) This antenna is designed for 27.555. I standby on channel 19 (27.185 MHz) and redesigned the
dimensions for this frequency. The graph above shows the predicted performance. 14dBi of forward gain should
roughly be equivalent to increasing power off my vertical from 4 to about 60 watts. The F/R ratio of 29dB should be
the equivalent of a 1kW station off the front of the beam, dropping to 1 watt when off the back. The F/B of 41.5 dB
(180 degrees or straight off the back) should be a difference of 8 thousand times. I think I got the numbers right. In
any case, according to the predictions, it should be more than satisfactory. Unfortunately, the Yagi for Windows
program only works for horizontal antenna modeling. Performance will be less in the vertical mode, I assume due to
interference from the mast being in line with the elements. If anyone has time to play with my numbers with an NEC
program for vertical mode, I'd love to hear from you. Email I use the radio for mostly local communications and
will be vertical 90% of the time.
The element lengths shown in the chart
are for half the actual element, as they're
symmetrical about the boom. Dia. 1
figures are fudged some. The entry here
is to compensate for how the element is
attached to the boom. In this case, a 3
inch long, split 1 inch tube is used to
reinforce the element where it's u-bolted
to the boom. Some variations in this
dimension didn't change much in the
predicted performance. Spacing
dimensions are for distance from the
element to the reflector. Note the extra
long distance between the two directors.
I added a 21 inch extension on the boom
to get the required length.
are for half the actual element, as they're
symmetrical about the boom. Dia. 1
figures are fudged some. The entry here
is to compensate for how the element is
attached to the boom. In this case, a 3
inch long, split 1 inch tube is used to
reinforce the element where it's u-bolted
to the boom. Some variations in this
dimension didn't change much in the
predicted performance. Spacing
dimensions are for distance from the
element to the reflector. Note the extra
long distance between the two directors.
I added a 21 inch extension on the boom
to get the required length.
I played with the dimensions until I got the
pattern on the right. The red graph is the
pattern for the direction the antenna's
pointed at and the green graph is
elevation.
I'm more concerned with the front to rear
ratio. This, again, is the predicted
performance for horizontal mode. For
direction finding, I believe this will work.
There's apparently a 20dB change in
signal strength when one of the antennas
changes polarization. I think if I can hear
a station, I can find the bearing to their
location.
pattern on the right. The red graph is the
pattern for the direction the antenna's
pointed at and the green graph is
elevation.
I'm more concerned with the front to rear
ratio. This, again, is the predicted
performance for horizontal mode. For
direction finding, I believe this will work.
There's apparently a 20dB change in
signal strength when one of the antennas
changes polarization. I think if I can hear
a station, I can find the bearing to their
location.
