tv antenna isolation transformer design help needed

[Andy]

Hello:

I would like to build a simple 1:1 galvanically isolating transformer,
for insertion into the 75 Ohm TV antenna path, for the band between
45MHz and 870 MHz.

There is a design available at

http://www.epanorama.net/documents/groundloop/antenna_isolator_building.html

but it uses a ferrite core. The core materials that I've come across are
not rated for more than 200MHz, so I have some doubts about the design.

Would a coreless winding work? Any help with the choice of the core
material, L, compensation network would be appreciated.

Thank you.

 

[Leon Heller]

http://www.kasamba.com/viewExpert.asp?conMemID=105725&Catid=1111&banID=2100

Hello:

I would like to build a simple 1:1 galvanically isolating transformer, for
insertion into the 75 Ohm TV antenna path, for the band between 45MHz and
870 MHz.

There is a design available at

http://www.epanorama.net/documents/groundloop/antenna_isolator_building.html

but it uses a ferrite core. The core materials that I've come across are
not rated for more than 200MHz, so I have some doubts about the design.

Would a coreless winding work? Any help with the choice of the core
material, L, compensation network would be appreciated.

It's a transmission line transformer, the material need only be suitable for
the lower end of the band.

Leon

 

[Andy]

http://www.kasamba.com/viewExpert.asp?conMemID=105725&Catid=1111&banID=2100



It's a transmission line transformer, the material need only be suitable for
the lower end of the band.

Leon

I am not very familiar with the transmission line transformers.
The device that I am looking for is a real transformer, with the energy
transmission by flux linkage. I understand that the frequency
increasing, the permeability of the core decreases, but the overall
reactance of the primary and secondary windings would still maintain the
impedance sufficiently high. But would not the losses in the core
severely hurt the efficiency?

-- Andy

 

[Winfield Hill]

Andy wrote...

The device that I am looking for is a real transformer, with
the energy transmission by flux linkage. ...

Why?

 

[Andy]

Andy wrote...



Why?

Well, this is to break the ground loops in the TV antenna feed. I
believe that I need to isolate down to tens of Hertz, and this looks to
me difficult to achieve with a kind of a UHF TLT. Maybe I am wrong. I am
ready to experiment.

 

[Fred Bloggs]

Well, this is to break the ground loops in the TV antenna feed. I
believe that I need to isolate down to tens of Hertz, and this looks to
me difficult to achieve with a kind of a UHF TLT. Maybe I am wrong. I am
ready to experiment.

Well- the epanorama page also shows the dual DC-block- you might try
that one. The capacitors are in series with the load so that the low
frequency cutoff becomes F= 1/(2 pi R C/2) and for 45MHz at 75 ohms this
becomes 100pF.

 

[gwhite]

I am not very familiar with the transmission line transformers.
The device that I am looking for is a real transformer, with the energy
transmission by flux linkage. I understand that the frequency
increasing, the permeability of the core decreases, but the overall
reactance of the primary and secondary windings would still maintain the
impedance sufficiently high. But would not the losses in the core
severely hurt the efficiency?

Transmission line transformers are far superior. You can block with caps as
Bloggs suggested. Sevic's book is good if you want to learn about them.

 

[Joerg]

Hi Andy,

In the olden days when money was sparse I used to do that the 'poor
man's way': Cut the coax, free a tip of the center conductor and scrape
the outer jacket away at about 3-4 inches back. Just a spot and not all
around. Then loop and solder center tip to shield there. Do this for
both and tape the whole chebang neatly on top of each other. When I
needed good lower VHF band performance I sometimes did this through a
43 material core. The only cost was the core at around 50 Cents or so.
No connectors. Oh, plus a beer since I usually did this to fix someone's
EMI problem when they had a flimsy TV in the neighborhood of a strong
transmitter.

It doesn't work with some aluminum quad-shield though because you often
can't really solder that stuff.

Regards, Joerg