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Two 1/4 wavelength transformers for one antenna and two RX-radios. Help my thinking

 

Hello,

I intend to use the same antenna for two RX-radios. I will lose 3 dB, but for the sake of the radios RX-input I intended to present each with roughly 50 ohms.

The frequency is about 90 MHz (not HAM-use). I made two 1/4 wave transformers with RG59 coax, taking VF of 0,66 in consideration. Measuring each one with the Nano and 50 on the outer side I get as anticipated about 110 ohm. Now the idea is to parallell the two 110 ohm sides and get around 55 ohm, this is where the antenna should be connected. Imagine the cable-setup as a V, with the vertex (bottom on the V) labeled A and the two upper parts labeled B and C.

When I connect 50 ohm dummies to B and C then I can measure about 50 ohm at A for 90 MHz. Now I wanted to see what one of the radios would feel so I connected 50 ohm dummies (actually 50 ohm pieces from calibration kits) at A and C. Now measuring at B I was expecting to see about 50 ohm. That was not the case, the result was about 140 ohm (X about 0) and SWR about 3 (of course). At about 150 MHz I got a SWR minimum.

Please help me to understand why the setup works one way (as e.g. to connect two phased antennas) but not the other way to connect one antenna to two radios.

73/Torbjorn/SM6AYM
 

The TV splitters to connect two TV's to the same antenna are very cheap and work just fine at that frequency.? ? They are a simple transformer, they have no impedance of their own.? So splitting 75 Ohms or 50 Ohms is no difference.? ? Don't like the PAL or F connectors, take out the little transformer and put it in your own box.? Kent

On Wednesday, May 4, 2022, 10:07:30 AM CDT, Torbj?rn Toreson <torbjorn.toreson@...> wrote:

Hello,

I intend to use the same antenna for two RX-radios. I will lose 3 dB, but for the sake of the radios RX-input I intended to present each with roughly 50 ohms.

The frequency is about 90 MHz (not HAM-use). I made two 1/4 wave transformers with RG59 coax, taking VF of 0,66 in consideration.? Measuring each one with the Nano and 50 on the outer side I get as anticipated about 110 ohm. Now the idea is to parallell the two 110 ohm sides and get around 55 ohm, this is where the antenna should be connected. Imagine the cable-setup as a V, with the vertex (bottom on the V) labeled A and the two upper parts labeled B and C.

When I connect 50 ohm dummies to B and C then I can measure about 50 ohm at A for 90 MHz. Now I wanted to see what one of the radios would feel so I connected 50 ohm dummies (actually 50 ohm pieces from calibration kits) at A and C. Now measuring at B I was expecting to see about 50 ohm. That was not the case, the result was about 140 ohm (X about 0) and? SWR about 3 (of course). At about 150 MHz I got a SWR minimum.

Please help me to understand why the setup works one way (as e.g. to connect two phased antennas) but not the other way to connect one antenna to two radios.

73/Torbjorn/SM6AYM
 


You're missing the resistor.
73 Andy, G4KNO.

On Wed, May 4, 2022 at 4:07 PM Torbj?rn Toreson <torbjorn.toreson@...>
wrote:

Hello,

I intend to use the same antenna for two RX-radios. I will lose 3 dB, but
for the sake of the radios RX-input I intended to present each with roughly
50 ohms.

The frequency is about 90 MHz (not HAM-use). I made two 1/4 wave
transformers with RG59 coax, taking VF of 0,66 in consideration. Measuring
each one with the Nano and 50 on the outer side I get as anticipated about
110 ohm. Now the idea is to parallell the two 110 ohm sides and get around
55 ohm, this is where the antenna should be connected. Imagine the
cable-setup as a V, with the vertex (bottom on the V) labeled A and the two
upper parts labeled B and C.

When I connect 50 ohm dummies to B and C then I can measure about 50 ohm
at A for 90 MHz. Now I wanted to see what one of the radios would feel so I
connected 50 ohm dummies (actually 50 ohm pieces from calibration kits) at
A and C. Now measuring at B I was expecting to see about 50 ohm. That was
not the case, the result was about 140 ohm (X about 0) and SWR about 3 (of
course). At about 150 MHz I got a SWR minimum.

Please help me to understand why the setup works one way (as e.g. to
connect two phased antennas) but not the other way to connect one antenna
to two radios.

73/Torbjorn/SM6AYM
 

On 2022-05-04 17:39:+0100, you wrote:


You're missing the resistor.
73 Andy, G4KNO
Very useful link. Thanks!

~R~
72/73 de Rich NE1EE
The Dusty Key
On the banks of the Piscataqua
 

Thanks Andy and Kent. Using the Wilkinson approach my splitter gives SWR 1.1 at the design frequency, quite broad. The input from Kent gave me the idea to make a splitter using two ferrite-toroidal transformers, winding prim/sek 5/7 turns would give the correct transformation, root of 2. Then I could place the transformers in a box together with three ccoax connectors. I will experiment with this setup.

73/Torbjorn
 

Regarding power splitters/combiners, check out the Gysel (rhymes with Diesel) combiner; its termination resistors are 50¦¸ and grounded, much easier to deal with than the floating 100¦¸ resistor of the Wilkinson type, especially when it comes to high power work.
73, Don N2VGU
 

On Wed, May 4, 2022 at 08:36 AM, KENT BRITAIN wrote:

The TV splitters to connect two TV's to the same antenna are very cheap and
work just fine at that frequency.? ? They are a simple transformer, they
have no impedance of their own.? So splitting 75 Ohms or 50 Ohms is no
difference.? ? Don't like the PAL or F connectors, take out the little
transformer and put it in your own box.? Kent
I tested a quality CATV splitter and found it worked quite well for 50 ohm applications. The Return Loss, S21 and Isolation graphs obtained using a NanoVNA are attached. I used F to BNC adapters (from Amazon) for these tests.

Roger

CommScope Port1 to port2 isolation.PNG
Commscope Splitter.jpg
CommScope Return Loss.PNG
CommScope S21.PNG

 

Hi Torbj?rn,

going back to your coax xable splitter:

When you have a signal source at A and two loads connected to B and C, the circuit works fine, because each 75? cable will transform its 50? load into 112.5?, and the two 112.5? inputs in parallel give 56.25?, close enough to the 50? the source expects.

When you have two phase-synchronous signal sources at B and C, the same will happen in reverse, and the circuit will work fine too.

But when you have a source at B, and loads at A and C, it will not work. The load at C will be transformed to 112.5? by that cable, this will appear in parallel with the load at A, giving 34.6?, and this will be transformed to 162.5? by the other cable.

Very simply said, your circuit isn't 3-way-symmetric. Ports B and C are identical and interchangeable, but port A is different. You always need to have either the source or the load on port A, and put identical things on B and C - two loads or two synchronized sources.

Manfred
 

Yet another splitter document. This one is from Mini-Circuits:
RT
 

Mini-Circuits has TONS of applications literature, and like their products, it's pretty much all top-notch. Look here:



and here:

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