I have a 6 meter beam with "plumbers delight" construction. I have not
been successful in making a 1/2 wave coax balun (T-match) get a proper
impedance match at 50.125 MHz center frequency. The driven element is
0.625" Outside diameter. The T-bars have adjustable shorting straps for
a max length of 14", with 3" spacing below the driven element.

The balun is made of RG-142 Teflon-coax, and is 81 inches long. The
velocity factor is 0.695, so the length is:

1/2 WaveLength (meters) = (150 * V)/F where F is in MHz.

Thus, the half-wave coax would be (150*0.695)/50.125 in meters.
Multiply by 39.37 inches per meter to get 81.8 inches for the coax length.

Any ideas why this did not do the job? I checked the coax for
opens/shorts and moved the T-bar shorting straps to no avail. The best
SWR was no better than about 3:1, even after adjusting the length of the
driven element.

I tested the driven element (both by itself and installed on the beam)
mounted on a temporary mast at about 10 feet to avoid most of the ground
effect.

Thanks,
Steve Bell KJ7OG

Steve,

Are you doing a T match or a Beta match? I looked at a Hygain 6 m beam, and
the shorting bars are set to 9-1/2 inches from the center. The driven
element is insulated at the center, and the beta rod is connected to the
boom at the center. The coax is connected to the driven element. As I
understand it, in a real T match the coax is connected to the T rod, but it
supposedly does not have as good an adjustment.

Tam/WB2TT
"Steve B." wrote in message
...
I have a 6 meter beam with "plumbers delight" construction. I have not
been successful in making a 1/2 wave coax balun (T-match) get a proper
impedance match at 50.125 MHz center frequency. The driven element is
0.625" Outside diameter. The T-bars have adjustable shorting straps for
a max length of 14", with 3" spacing below the driven element.

The balun is made of RG-142 Teflon-coax, and is 81 inches long. The
velocity factor is 0.695, so the length is:

1/2 WaveLength (meters) = (150 * V)/F where F is in MHz.

Thus, the half-wave coax would be (150*0.695)/50.125 in meters.
Multiply by 39.37 inches per meter to get 81.8 inches for the coax length.

Any ideas why this did not do the job? I checked the coax for
opens/shorts and moved the T-bar shorting straps to no avail. The best
SWR was no better than about 3:1, even after adjusting the length of the
driven element.

I tested the driven element (both by itself and installed on the beam)
mounted on a temporary mast at about 10 feet to avoid most of the ground
effect.

Thanks,
Steve Bell KJ7OG

Thanks Tam, for the reply.
I am using a T-match, with the driven element NOT insulated at the
center, similar to the T-match used on the M-squared yagis for VHF. I
cannot get a match no matter how much I move the shorting straps on the
T-bars.
Steve

Tarmo Tammaru wrote:
Steve,

Are you doing a T match or a Beta match? I looked at a Hygain 6 m beam, and
the shorting bars are set to 9-1/2 inches from the center. The driven
element is insulated at the center, and the beta rod is connected to the
boom at the center. The coax is connected to the driven element. As I
understand it, in a real T match the coax is connected to the T rod, but it
supposedly does not have as good an adjustment.

Tam/WB2TT
"Steve B." wrote in message
...

I have a 6 meter beam with "plumbers delight" construction. I have not
been successful in making a 1/2 wave coax balun (T-match) get a proper
impedance match at 50.125 MHz center frequency. The driven element is
0.625" Outside diameter. The T-bars have adjustable shorting straps for
a max length of 14", with 3" spacing below the driven element.

The balun is made of RG-142 Teflon-coax, and is 81 inches long. The
velocity factor is 0.695, so the length is:

1/2 WaveLength (meters) = (150 * V)/F where F is in MHz.

Thus, the half-wave coax would be (150*0.695)/50.125 in meters.
Multiply by 39.37 inches per meter to get 81.8 inches for the coax length.

Any ideas why this did not do the job? I checked the coax for
opens/shorts and moved the T-bar shorting straps to no avail. The best
SWR was no better than about 3:1, even after adjusting the length of the
driven element.

I tested the driven element (both by itself and installed on the beam)
mounted on a temporary mast at about 10 feet to avoid most of the ground
effect.

Thanks,
Steve Bell KJ7OG

Sounds fun.

Did you try adjusting the length of the driven element, if it is
adjustable? Just a wee bit, near that 3:1 SWR point?

Also, seems you might be better served with a 1:1 coil of coax as a
choke, unless I misunderstand. You should be able to get a direct match
to the coax.

Don't know for sure, but it seems like the driven element needs to be a
wee bit short for gamma type matches. Hmmm, I feel an Uda attack coming on.

(Hey lets give the grad student some credit! He probably did all the
real work, while Yagi was back in the lab, reading Uda's notes and
tossing back saki. That's how it works here in the US, aina, 'cept they
drink Chivas...)

good luck!

73, w4kaz

Steve B. wrote:
I am using a T-match, with the driven element NOT insulated at the
center, similar to the T-match used on the M-squared yagis for VHF. I
cannot get a match no matter how much I move the shorting straps on the
T-bars.

This is the kind of problem that is relatively easy to solve with an
antenna analyzer.
--
73, Cecil http://www.qsl.net/w5dxp



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"Steve B." wrote in message
...
Thanks Tam, for the reply.
I am using a T-match, with the driven element NOT insulated at the
center, similar to the T-match used on the M-squared yagis for VHF. I
cannot get a match no matter how much I move the shorting straps on the
T-bars.
Steve
Steve,

I have an M-Squarec 432 antenna. The T rod, if that is what it is, is about
3/4 the length pf the drivem element. That would suggest your 9-1/2 inches
at 50 MHz is way too short; about right for a Beta match, though.

Tam/WB2TT

Cecil and w4KAZ:

Thanks guys, for the replies. Yes, I used the antenna analyzer and
played with the driven element length, and the T-bar shorting straps to
get the "best" result at swr=3.

For additional checking, I removed the driven element, and replaced the
T-bars with a 200 ohm resistor. This should give 50 ohms at the
resonance of the 4:1 1/2 wave coax balun. Well, suprise, the coax did
not have the advertised velocity factor, so the resonance (as verified
by an MFJ 259 analyzer) was at 51.45 MHz, instead of the desired 50.125
MHz. This is a minor annoyance, since the SWR at 50.125 would still be
acceptable.

My next experiment: make the T-bars longer, since the driving point Z of
the driven element at the center is only about 20 Ohms. By tapping
farther out on the element, the differential Z would (hopefully)
approach the desired 200 OHms at resonance.

I will keep you posted.

Steve
KJ7OG
------------------------------

Cecil Moore wrote:
Steve B. wrote:

I am using a T-match, with the driven element NOT insulated at the
center, similar to the T-match used on the M-squared yagis for VHF. I
cannot get a match no matter how much I move the shorting straps on
the T-bars.


This is the kind of problem that is relatively easy to solve with an
antenna analyzer.

Tarmo:
Yes, it does appear that the T-bars are too short. I looked up a 10
Meter T-match on K1ttt.net and he uses :
Rod dia = 0.5"
Spacing = 4"
Length = 24"
Lead length = 3"

This would scale to 6 meters to about 15" length for the T-bars. I will
build a new set that is at least that long and give it a try, probably
Wed. Jan. 7
Thanks,
Steve

Tarmo Tammaru wrote:
"Steve B." wrote in message
...

Thanks Tam, for the reply.
I am using a T-match, with the driven element NOT insulated at the
center, similar to the T-match used on the M-squared yagis for VHF. I
cannot get a match no matter how much I move the shorting straps on the
T-bars.
Steve

Steve,

I have an M-Squarec 432 antenna. The T rod, if that is what it is, is about
3/4 the length pf the drivem element. That would suggest your 9-1/2 inches
at 50 MHz is way too short; about right for a Beta match, though.

Tam/WB2TT