Topband: 75 ohm RG6 to 50 Ohm radio

donovanf at donovanf at
Wed Sep 28 11:02:55 EDT 2016

Hi Grant, 

Regarding your comment about the isolation transformer 
properties of a transformer wound on a binocular core (or on 
a toroidal core), their common mode choking performance is 
very inferior to common mode chokes such as those 
documented by K9YC. 

Relatively compact and inexpensive receiving common mode 
chokes are easily constructed using 75 ohm RG-179 or 
50 ohm RG-174 miniature coaxial cable. 


----- Original Message -----

From: "Grant Saviers" <grants2 at> 
To: "Ed Stallman" <ed.n5dg at>, topband at 
Sent: Wednesday, September 28, 2016 2:45:21 PM 
Subject: Re: Topband: 75 ohm RG6 to 50 Ohm radio 

One thing that W8JI clarified for me in winding binocular cores is what 
is a "turn". Normally, a wire thru a toroid hole is a "turn". A 
binocular core is two toroids in a single piece of ferrite. Tom 
proposed calling a wire in one side of a binocular core a "pass" and 
thru both sides a "turn". Thus, 2.5 "turns" are possible with 5 
"passes". Usually, a "turn" by authors means 2 "passes" but there are 

With the different windings coming out of opposite ends there will be a 
little less stray coupling and it makes it easier to connect them to 
their respective connectors. If there is no ground path between the 
conductors then you have an isolation transformer which can reduce noise 

I use small watertight plastic boxes I found on ebay for all sorts of 
receive antenna projects, very well made. 

If you use some 30ga insulated wire wrap wire, it is less fragile 
against the ferrite and has a higher isolation voltage between the 
windings than enamel insulation. A little RTV or hot melt keeps the 
core secured to the box. 

Grant KZ1W 

On 9/27/2016 20:46 PM, Lee STRAHAN wrote: 
> Yes Ed, You have the right numbers. It won't matter which windings are first or last. You can even wind 4 then wind 1 more and use it as an autotransformer where the 1 turn is added to the 4 for 5. Any of these will work fine at these TOP Band and higher frequencies. This ratio is not perfect but will work just fine. It would be 50 to 78 ohms which in our world is close enough. 
> Let us know how your new 8 circle works. Maybe even compare it to your Hi-Z 4-square. 
> Lee K7TJR 
> -----Original Message----- 
> From: Topband [mailto:topband-bounces at] On Behalf Of Ed Stallman 
> Sent: Tuesday, September 27, 2016 8:35 PM 
> To: topband at 
> Subject: Topband: 75 ohm RG6 to 50 Ohm radio 
> My receive antenna is 75 Ohm RG6 connected to my radio and I want to wind a 2873000202 binocular core to step 75 to 50 Ohm's . I seen this info on W8JI web page but can't seem to find it again. 
> I do remember it's 4 turns 50 Ohm and 5 turns 75 Ohm and all wires come out the same side of the core . Am I correct so far? 
> Does it make a difference which winding I wind first ? 
> Thanks in advance , Ed N5DG 
> --- 
> This email has been checked for viruses by Avast antivirus software. 
> _________________ 
> Topband Reflector Archives - 
> _________________ 
> Topband Reflector Archives - 

Topband Reflector Archives - 

More information about the Topband mailing list