Combining SW & FM Antenna Coax Leadins

[cod]

I have a SWL antenna that has a 75 ohm coax lead out of the switch box/tuner to an 1/8' audio plug. I also have a simple folded dipole for FM with a balun so it too has a 75 ohm lead out to an 1/8" audio plug. Each time I listen to SW I have to manually insert one plug into the radio's antenna jack and when I switch to FM, it's pull the first plug and insert the second. What type of matching device can I build to combine the signals from the two coax leadins into one coax output to the audio plug which I could then leave inserted in the radio indefinitely? I would prefer that it be passive yet not degrade the signal from either antenna.

I understand that Blonder-Tongue has a "Multiplexer" (MDX-75F), also called a "Band Combiner" that will do the job, but I'd like to build my own.

Anybody have a schmatic of how to do it?

Thanks for any help.

Charles Daniell

 

[jbartos]

Suggestion: Check

http://www.northamericancable.com/

http://my.ece.ucsb.edu/yorklab/Projects/Amplifiers/X-band/MAFET X-band Combiner.htm

http://www.klmicrowave.com/pdf/txcombiners/txcombiners.pdf

http://www.blondertongue.com/WebPages/Catalog/pdf/Cat_2001/Index_Cat_2001_S.pdf

etc. for more info

 

If I were in your shoes, I would do the following:

1) Check to ensure that the folded dipole provides sufficient increase in FM reception to be worth the effort. Do you get more stations with the folded dipole than you do just using the SW antenna system ? Are you using the FM just for local reception ?

2) Consider just using a simple two-position switch if required.

Good luck.

 

[cod]

VE1BLL

Thanks for your thoughts. Yes, the FM station is a little 500W affair run by high school kids that has the best music in the world (for an old timer. The station can't be received without the dipole on most days and the SW antenna on FM doesn't cut it. So I have to have them both.

Yes, the switch is so far the only solution I've heard of. I just thought a "combiner" or some such thing might be readily available.

What I'm somewhat surprised at is that nobody has offered some kind of schematic on how to build one. The Blonder Tongue device would be just fine but one of its inputs is tuned to the wrong frequency. It must be made up of all passive parts so it can't be comprised of anything but coils and transformers, maybe some capacitors.

Yep, I think I'll wire in a switch, a simple solution.

Thanks again

 

[jbartos]

Suggestion: Click on the second link in my above posting

http://my.ece.ucsb.edu/yorklab/Projects/Amplifiers/X-band/MAFET X-band Combiner.htm

and then on
X-band wave-guide-based combiner systems
to get some idea about the Circuit Topology
Please, notice that there are referenced papers that include useful information for the production, e.g. test setup. All what this needs is to do more clicking on the links and reading. I click on several hundreds links every day.

 

[rodar]

I have wondered if this would work.
If you bring the shortwave antenna lead
to the feed point of the folded dipole
just connect the two of them. The idea is
that the folded dipole is a high impedance load
to the shortwave signals and should not
load that signal. The key is for there to
very little cable between the shortwave line
and the feed point of the folded dipole.
As for the shortwave antenna loading the
folded dipole, it will provide a high impedance
to the fm signal. The only unknown would be
the the length of coax between the fm and shortwave
antenna should be a multiple of 1/2 wavelength
at fm frequencies thereby creating the open back
at the folded dipole.
Just food for thought
This would only work if both antennas are matched
to 50 ohm.

 

The FM folded dipole might well effectively 'short out' the SW antenna. I don't believe that it would be 'open circuit' at SW frequencies.

To help this be clear, imagine a folded dipole for microwave frequencies (perhaps about the size of a paper clip). If such a small antenna were connected in parallel with a SW antenna, it would be basically the same as a dead short across the SW feed point. The FM-band folded dipole is along the same lines, but not as extremely obvious as the paper clip example.

On the other hand, a normal FM-band dipole (not folded) would certainly be invisible to a parallel connected SW antenna.

I'd still just go with a switch.

 

[cod]

Decision made! I bought a TV "Cable-Antenna" Switch at HOme Depot and it works as expected.

I tried to see if there was any diffeence in the signal to the radio by first using the switch and then reconnecting without the switch in the circuit.

I saw (heard) no difference in either the FM or the SW signal. Maybe well calibrated instruments might say differently but I'm happy with the outcome.

Thanks to all for the help.

 

[jbartos]

Suggestion: Visit

http://members.tripod.com/~clearRX/RDFUNIT.HTM#(4)

for more info