Towertalk
[Top] [All Lists]

Re: [TowerTalk] common mode chokes, baluns and multiband doublets

To: Bert <balmemo@sympatico.ca>, towertalk@contesting.com
Subject: Re: [TowerTalk] common mode chokes, baluns and multiband doublets
From: Grant Saviers <grants2@pacbell.net>
Date: Mon, 11 Dec 2017 17:04:49 -0800
List-post: <mailto:towertalk@contesting.com>
It seems there is a good solution to a single common mode choke for balanced lines over all of HF. See the article in November 2017 RadCom (RSGB) "An effective, easily made HF Balun". G3LNP describes a 4:1 balun for all band doublets fed with REAL open wire feeders, with CM rejection on the order of 40db.

Additionaly, consider G3TXQ's measurement of punched ladder line losses. http://www.karinya.net/g3txq/wet_ll/

I evangelize using real open wire, easy to make or buy. Examples:

http://trueladderline.com/w7fg-design-600-ohm-open-wire-feedline/
http://www.73cnc.com/product_p/ls100.htm

Grant KZ1W



On 12/11/2017 16:15 PM, Bert wrote:
We're talking about a common mode choke, not a balun/transformer.

Bert, VE3NR



On 12/11/2017 7:08 PM, Wes N7WS wrote:
Actually that's not true. A lot of baluns are wound with balanced transmission line.

Wes. N7WS

Sent from my iPhone

On Dec 11, 2017, at 3:49 PM, Bert <balmemo@sympatico.ca> wrote:

Hi Bill,

Choking the window line is not a good idea - you'll choke the signals too. Common mode chokes are mainly used for coax cables and multi-conductor cables. I'm sure you'll get many more detailed replies from the experts.

Good Luck!

Bert, VE3NR



On 12/11/2017 4:33 PM, k7wxw wrote:

Yesterday the wind brought down my doublet, which I mostly see as an upgrade opportunity, at least once I finish pulling tangles of wire out of trees. While a window line fed doublet is a well-known beast, and the one I had up worked okay, I hope the new version will perform better by taking into account my particular installation limitations. As I worked on the design, I came up with a couple of questions...

I apologize if this is too much detail, or whatever.

I am putting up a centerfed doublet to cover 80 through 10 meters, focusing on 80, 40, 30, and 20 meters. I plan on feeding it with a random length of 450 ohm window line. I have a Johnson Matchbox and Dentron Supertuner and can use either to match the antenna to my rigs. My antennas come to a common connection panel, which is mounted on an ground rod. I rarely run more than 25W and never more than 150W.

For a number of reasons - including the fact that it won't be quite level, it is only up about 30 feet and part of it runs over the house - the antenna will not be balanced. If I install a 1:1 current choke built using two stacked FT-140-43 toroids at the antenna feed point, is it likely to survive given the wide variation in feedpoint impedance over the operating frequency range?

I originally planned on using a Morgan balanced line lightning supressor. Unfortunately, it is only rated for input and output impedances of 300 to 600 ohms, a condition that is impossible to meet in a multiband antenna. My alternative is using a 4:1 balun, followed by a polyphase lightning suppressor. Same question about this balun (stacked FT-140-43)... how will it do given the variation in load impedance over the operating range?

The 4:1 balun at the base of the feedline has the added advantage that I can avoid running window line into the house (I am doing that now) which is a PITA.

Bonus question: I know that expecting baluns and common mode chokes to work well over 3 to 30 Mhz is, well, optimistic. Will I get better common mode suppression at the feedpoint by using two chokes, material chosen to optimize different portions of the operating range?

I have read K9YC's guides, along with a lot of material from W8JI and others but I haven't quite figured out how to figure out the answers to my questions.

Thanks for any help and 73 de bill K7WXW



_______________________________________________



_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________



_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________



_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk


_______________________________________________



_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk


_______________________________________________



_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk

<Prev in Thread] Current Thread [Next in Thread>