Hi all,

When co-phasing two identical two meter yagis, is it necessary to use
the "power dividers" sold by the antenna manufacter? If two odd
multiples of 1/4 wavelength 75 Ohm coax is carefully constructed and
run into a T connector, won't this achieve the same thing? Is there a
real peformance difference?

Thanks,
Scott, WU2X

All you need for this special case are two pieces of coax which are the
same length and a tee connector. Yes, there might be a performance
difference -- "power dividers" are likely to add unnecessary loss to the
system, reducing your signal strength.

"Power dividers" aren't the right solution for any antenna phasing
problem. See Chapter 8 of the _ARRL Antenna Book_ for more information.
And feeding two elements or phased antennas with just two pieces of coax
is usually more complicated than cutting them to the difference in phase
angle, as explained in the _Antenna Book_ and
http://eznec.com/Amateur/Articles/Simpfeed.pdf. But yours is a special
case that's easy to do.

Roy Lewallen, W7EL

wrote:
Hi all,

When co-phasing two identical two meter yagis, is it necessary to use
the "power dividers" sold by the antenna manufacter? If two odd
multiples of 1/4 wavelength 75 Ohm coax is carefully constructed and
run into a T connector, won't this achieve the same thing? Is there a
real peformance difference?

Thanks,
Scott, WU2X

wrote in message
ups.com...
Hi all,

When co-phasing two identical two meter yagis, is it necessary to use
the "power dividers" sold by the antenna manufacter? If two odd
multiples of 1/4 wavelength 75 Ohm coax is carefully constructed and
run into a T connector, won't this achieve the same thing? Is there a
real peformance difference?

Thanks,
Scott, WU2X

Hi Scott

That pair of 75 ohm transformers will work. I'd suggest that you
consider only 1/4 wavelength for the 70 ohm transformer in each line from
the 50 ohm Yagis. Additional transformer length will effect impedance
bandwidth.
I'd suggest using the "power dividers" unless $$cost is a consideration.

Jerry

If the antennas are to be spaced more than a 1/2 wavelength (free
space), how can I use a 1/4 wavelength of 75 Ohm coax? I guess I would
have to splice in a 1/2 wavelength multiple of 50 Ohm coax between the
75 ohm coax and the T connection to make up the length.

I only need about 500Khz of bandwidth, so this might not be a concern.
But this was another question I was going to raise. Is there anyway to
calculate the bandwidth that I would see from a 1/4 wavelength
matching section? I guess that is a real easy one to test in real life
though.

Thanks for all the responses!

-Scott, WU2X


On Mar 8, 1:28 pm, "Jerry Martes" wrote:


I'd suggest that you
consider only 1/4 wavelength for the 70 ohm transformer in each line from
the 50 ohm Yagis.

wrote in message
ups.com...
If the antennas are to be spaced more than a 1/2 wavelength (free
space), how can I use a 1/4 wavelength of 75 Ohm coax? I guess I would
have to splice in a 1/2 wavelength multiple of 50 Ohm coax between the
75 ohm coax and the T connection to make up the length.

I only need about 500Khz of bandwidth, so this might not be a concern.
But this was another question I was going to raise. Is there anyway to
calculate the bandwidth that I would see from a 1/4 wavelength
matching section? I guess that is a real easy one to test in real life
though.

Thanks for all the responses!

-Scott, WU2X


Hi Scott

If it is OK to assume the antenna's input impedance is close to 50 ohms.
Any length of 50 ohm line can be installed to a location where the 1/4 wave
transformers can be installed.
It may be confusion for me to add that I have connected two 50 ohm coaxes
in series to make them look like 100 ohms at the junction. I didnt think
the joining of coax center conductor to the outer conductor of another was a
good idea till I did it (after being advised by Mr Richard Clark).
Series-ing coax works quite well at 2 meters when a couple ferrite sleeves
are used to minimize currents on the outside of the shield. That allows
only one 75 ohm matching transformer (which isnt much of an advantage in
your case).

Jerry

Oh yes, what was I thinking. From the antenna feedpoint, I can run two
equal length pieces of 50 ohm coax to the 1/4 wavelength matching
sections into the T connector - as you just said.

But it will be much more convenient for me to just use an entire run
of 75 Ohm coax, but as I said before, now you have me curious about
the effect on bandwidth - I will have a total of 1.75 wavelengths of
75 Ohm coax per leg.

-Scott, WU2X



On Mar 8, 2:31 pm, "Jerry Martes" wrote:


If it is OK to assume the antenna's input impedance is close to 50 ohms.

wrote in message
ps.com...
Oh yes, what was I thinking. From the antenna feedpoint, I can run two
equal length pieces of 50 ohm coax to the 1/4 wavelength matching
sections into the T connector - as you just said.

But it will be much more convenient for me to just use an entire run
of 75 Ohm coax, but as I said before, now you have me curious about
the effect on bandwidth - I will have a total of 1.75 wavelengths of
75 Ohm coax per leg.

-Scott, WU2X


Hi Scott

How much do you like Smith Charts?? It would be very easy to plot, for
three frequencies, the impedance at the "far end" of the 75 ohm lines.
That seems like a 5 minute job. Then you will know the amount of error
introduced at the edges of your bandwidth. You'd know the impedance at the
upper, mid and lower frequencies. It may be that a couple wavelengths of 75
ohm line may not introduce enough error to be concerned with.

Jerry

Scott wrote:
If the antennas are to be spaced more than a 1/2 wavelength (free
space), how can I use a 1/4 wavelength of 75 Ohm coax? I guess I would
have to splice in a 1/2 wavelength multiple of 50 Ohm coax between the
75 ohm coax and the T connection to make up the length.

Use any lengths of 50-ohm coax, and then connect them using the
"combiner" or "power divider" of your choice.

In practice the divider/combiner will be simply a quarter-wave impedance
transformer associated with a T-piece. The 75-ohm coax version will
transform each leg from 50 to 100 ohms (ideally you'd need 70.7-ohm coax
for that) and then the T-piece connects those in parallel to give 50
ohms. Alternatively, you can use the type of divider/combiner that
directly parallels the two 50-ohm lines, and then transforms the
resulting 25-ohm impedance back up to 50 ohms through a quarter-wave of
home made 35-ohm hardline.

There's not a lot of practical difference between the two methods,
except that the first one requires you to cut and terminate two sets of
equal lengths of coax. The second one is easier electrically, but
requires some basic metalworking skills.

I only need about 500Khz of bandwidth, so this might not be a concern.
But this was another question I was going to raise. Is there anyway to
calculate the bandwidth that I would see from a 1/4 wavelength
matching section? I guess that is a real easy one to test in real life
though.

If you need only 500kHz bandwidth at 144MHz, that will not be a concern.


--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

wrote:
Oh yes, what was I thinking. From the antenna feedpoint, I can run two
equal length pieces of 50 ohm coax to the 1/4 wavelength matching
sections into the T connector - as you just said.

But it will be much more convenient for me to just use an entire run
of 75 Ohm coax, but as I said before, now you have me curious about
the effect on bandwidth - I will have a total of 1.75 wavelengths of
75 Ohm coax per leg.

I just saw the above, after having posted my first reply.

That wouldn't be a smart idea. Compared with either of the solutions
that uses only a single quarter-wavelength for the impedance
transformation, running two mismatched legs to 1.75 wavelengths will
increase the rate of change of phase with frequency by a factor of
1.75/0.25 = 7.

In that case, bandwidth might indeed become a problem, and phase
matching a big problem.


--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

See comments below.

wrote:
Hi all,

When co-phasing two identical two meter yagis, is it necessary to use
the "power dividers" sold by the antenna manufacter? NO!, use ODD
multiples of a 1/4 wavelength (dont forget the
velocity factor) of 75 ohm coax (for 50 ohm ants)!


If two odd
multiples of 1/4 wavelength 75 Ohm coax is carefully constructed and
run into a T connector, won't this achieve the same thing? Is there a
real peformance difference? Only in the small difference of loss in the
coax vs. the loss in the (devider+ coax lengths)
to the antennas! And ALSO, consider that the
lengths from a power devider must be identical
(to the antennas, and fed in PHASE), tho they are
NOT limited to the
(Odd 1/4 wavelengths, of a impedence matching
setup! Also, If you use , say 1/4 and a 3/4 wave
pieces for the 75 ohm example, (it works), BUT
remember the feed on antenna left right, and
the other antanna Right to LEFT! (180 degree
phase shift)! JIM NN7K

Thanks,
Scott, WU2X