formula stacking two dipoles
Michel de Ruiter
right to use 3/4 quarter wave?
Regards,
PD1AIW
Richard Harrison
"Who can give me the formula for stacking two dipoles with 75 ohm coax?
Is it right to use 3/4 quarter wave?"
There are several "Lazy H" arrays. See page 8-46 in the ARRL Antenna
Book or some similar book.
One answer for stacking two 1/2-wave horizontal dipoles is to separate
them by about 1/2-wave vertical distance and parallel in the same
vertical plane, then hang them at great height.
Coax without a balun may become a radiator too. But, the two dipoles
need to be driven in the same phase. Equal lengths of coax will get that
right. If these are 1/4-wave or 3/4-wave they can be "Q-sectons" to
elevate the dipole element impedance to twice the impedance of the coax
impedance of the longer length you want to use to match your radio. When
paralleled through a Tee connector you can have the right impedance.
The formula for a "Q-section" impedance is its impedance is equal to the
square root of the product of the two impedances to be matched.
Another approach is to use equal lengths of coax matching the antenna
elements. Then, install a "Q-section" between the Tee and the longer
line which matches your radio.
A better solution may be to use ladder-line with your balanced antenna
array. You could start by examining the web pages of Cecil Moore, W6RCA.
You will find numerous postings by Cecil in this newsgroup which contain
his webpage addresses.
Best regards, Richard Harrison, KB5WZI
Mark Keith
>
> Another approach is to use equal lengths of coax matching the antenna
> elements. Then, install a "Q-section" between the Tee and the longer
> line which matches your radio.
This is the method I used when I ran a phased array of two parallel
dipoles on 40m. I used equal lines to each antenna, and then switched in
added sections to steer the pattern. Mine was very casual as far as the
phasing. I just put it up, and played around with different sections
until I found the directions I liked the best. The antenna worked quite
well. Maybe 20 db or more difference when switching directions to
someone 1000 miles away. You can also use L-C phasing units. But the
orignal poster really isn't giving enough info on the antenna to tell.
Sounds like a vertical array which I think would need a to be fed either
from the center of the antenna with a normal feeder, or if fed at the
base as he describes would need a transposed feeder between the two
elements. I'm not sure how well coax would act as a phasing line if you
simply reversed the element connections between the two. Ladder line
would be easier to do this than 75 ohm coax, and is usually the norm. I
guess you could use ladder line as the phasing line in the center, and
feed with 75 ohm coax at the base. Might need a little matching. With
1/2 wl spacing , the impedance around 100 ohms or so. You can use
anything from about 3/8 wl to 3/4 wl. 5/8 wl gives the most gain, but
1/2 to 3/4 will do well. An advantage to the ladder line feeder instead
of coax would be multiband use. MK
--
http://web.wt.net/~nm5k
Michel de Ruiter
More info about the situation:
We are testing for a local repeater. As I wrote we like to use two stacked
vertical dipoles. As we don`t wanna use commercial antenna systems. It are
vertical dipoles with a gamma match feeded in the middle and we gonna use it
on two meters.
We wanna stack the dipoles for getting more gain. Now there are farious ways
to stack those dipoles. We understood that the easiest way to do it is using
75 ohm coax with 3/4 way length to the dipoles feedeed in the middle. Is
this a good way or can we do it better with 5/8 wave for getting more gain?
Looking forward for any reply.
With kind regards,
Michel de Ruiter
PD1AIW / JO21PU
"Mark Keith" <nm...@wt.net> schreef in bericht news:3B0AC4...@wt.net...
Mark Keith
>
> Sirs,
>
> More info about the situation:
>
> We are testing for a local repeater. As I wrote we like to use two stacked
> vertical dipoles. As we don`t wanna use commercial antenna systems. It are
> vertical dipoles with a gamma match feeded in the middle and we gonna use it
> on two meters.
>
> We wanna stack the dipoles for getting more gain. Now there are farious ways
> to stack those dipoles. We understood that the easiest way to do it is using
> 75 ohm coax with 3/4 way length to the dipoles feedeed in the middle. Is
> this a good way or can we do it better with 5/8 wave for getting more gain?
>
> Looking forward for any reply.
>
> With kind regards,
>
> Michel de Ruiter
> PD1AIW / JO21PU
I'm not familiar with feeding them with a gamma match, but that doesn't
mean it isn't done. If you feed each 1/2 wave from the end the impedance
will be very high. Normally you would probably use a low loss coil and
cap to match, but like I say, I've never made anything quite like that.
The only ones I've used were base fed cushcraft ringo ranger style using
a phasing stub between the elements. If this is for a repeater, I would
make separate dipoles ala 4 pole array or some such setup. In this
manner you center feed dipoles with equal length coax. This type antenna
is well suited for a repeater as you can set it up omnidirectional, or
for gain in a few different patterns depending on where you mount the
elements. Cushcraft used to make these, but quit I'm pretty sure. But I
do still have the PDF file for this antenna. And I would think it's
still at the cushcraft web site. The pdf file is a manual with all info
needed to make one. This type antenna would be a much better bet I think
than trying to get a single piece collinear going, which can be prone to
many problems. These 4 poles were commonly used for repeaters, so I know
they would be well suited. You could also make only a 2 pole. And being
they are mounted separate, you can optimize the spacing for the most
gain easily. And being center fed, the feed impedance is low and easy to
deal with using coax. MK
--
http://web.wt.net/~nm5k