I notice my combo antennas do NOT have a splitter in joining the two
antennas.

If I add a UHF antenna to my VHF antenna - 4 feet apart, MUST I use a
balum or joiner/splitter 300 to 75 bla bla bla.

What is the result if I just join them with 300 ohm twin lead and
then balum to 75 coax for the run to the TV?

I notice my combo antennas do NOT have a splitter in joining the two
antennas.

If I add a UHF antenna to my VHF antenna - 4 feet apart, MUST I use a
balum or joiner/splitter 300 to 75 bla bla bla.

What is the result if I just join them with 300 ohm twin lead and
then balum to 75 coax for the run to the TV?

When you connect the ends of two cables together to one feed cable, you get
a different impedance. If that new impedance is what you want to terminate
the feeder with, then you have an impedance match. If it is not a match then
the mismatch will result in a standing-wave back to the antenna. The
standing wave will create an impedance gradient (repeated at 1/2-wave x VF
intervals) along the cable, and that gradient could also be used to match to
a wanted impedance.

If you terminate a 300-Ohms cable from your antenna with a dead-short, then
the characteristic impedance of the cable again will vary along its length,
becoming high/low at 1/4-wave intervals - HLHLHLHLHLHL... etc. In principle
you can connect a 75-Ohm cable at the correct point in the 300 Ohm cable and
get a good match.

An antenna may also have a low impedance within its resonance band, but a
high impedance at other frequencies. Two such antennas can be paralleled so
that power is only transferred to the antenna having a low impedance. This
technique can be used at VHF/UHF/SHF/FHF, etc.

All these techniques can often be compounded with each other to form quite
complex antenna feed solutions.

On Thu, 05 Jul 2007 17:17:50 GMT, Charles wrote:
I notice my combo antennas do NOT have a splitter in joining the two
antennas.

If I add a UHF antenna to my VHF antenna - 4 feet apart, MUST I use a
balum or joiner/splitter 300 to 75 bla bla bla.

What is the result if I just join them with 300 ohm twin lead and
then balum to 75 coax for the run to the TV?

When you connect the ends of two cables together to one feed cable, you get
a different impedance. If that new impedance is what you want to terminate
the feeder with, then you have an impedance match. If it is not a match then
the mismatch will result in a standing-wave back to the antenna. The
standing wave will create an impedance gradient (repeated at 1/2-wave x VF
intervals) along the cable, and that gradient could also be used to match to
a wanted impedance.

If you terminate a 300-Ohms cable from your antenna with a dead-short, then
the characteristic impedance of the cable again will vary along its length,
becoming high/low at 1/4-wave intervals - HLHLHLHLHLHL... etc. In principle
you can connect a 75-Ohm cable at the correct point in the 300 Ohm cable and
get a good match.

An antenna may also have a low impedance within its resonance band, but a
high impedance at other frequencies. Two such antennas can be paralleled so
that power is only transferred to the antenna having a low impedance. This
technique can be used at VHF/UHF/SHF/FHF, etc.

All these techniques can often be compounded with each other to form quite
complex antenna feed solutions.

Or, you can realize that both your "all"-channel antennas will present
300 ohms to the feedline at only a few (damn few) select frequencies.
At other frequencies the impeadance will be gawd-knows-what greater or
gawd-knows-what less.

So, for a multi-channel use such as the OP's, just bring short runs of
300 twinlead from both antennas _together_ -- into a 300ohm-to-75ohm --
balun and move on with your life.

Jonesy
--
Marvin L Jones | jonz | W3DHJ | linux
38.24N 104.55W | @ config.com | Jonesy | OS/2
*** Killfiling google posts: http://jonz.net/ng.htm

In article ,
ValveJob wrote:

What is the result if I just join them with 300 ohm twin lead and
then balum to 75 coax for the run to the TV?

VJ-

I'd say the result is unpredictable. If you're referring to standard
commercial television, some TV channels will work great, some won't.

Why not use a UHF/VHF combiner network? They used to be sold in
electronics stores like Radio Shack, and come in a couple of versions
including all 300 Ohm, and 300 Ohm antennas into a 75 Ohm set.

Fred
K4DII

Why not use a UHF/VHF combiner network? They used to be sold in
electronics stores like Radio Shack, and come in a couple of versions
including all 300 Ohm, and 300 Ohm antennas into a 75 Ohm set.

You can´t do that. Its too easy! The solutions HAS to be complicated. That
is the first rule of homebrew. How else do you intend to impress
(uneducated) friends?