[TowerTalk] Transformer for 50 Mhz

D. Scott MacKenzie kb0fhp at comcast.net
Wed Mar 15 22:06:17 EST 2006

I was looking for a narrow band solution, and repeat it using different
dimensions for 6M, 2M and 432...But I like the taper feedline solution -
very nice.  Over what length must it taper?


-----Original Message-----
From: Jim Lux [mailto:jimlux at earthlink.net]
Sent: Wednesday, March 15, 2006 7:54 PM
To: kb0fhp at comcast.net; TowerTalk at contesting.com
Subject: Re: [TowerTalk] Transformer for 50 Mhz

At 02:23 PM 3/15/2006, D. Scott MacKenzie wrote:
>I am looking for plans for a transformer/balun to couple 50 ohm at 50Mhz to
>300 ohm.  The purpose is to load a TV antenna, allowable by HOA, on 6M.  I
>have tested this antenna, using the standard transformer from 300 ohm to 75
>ohm, using a MFJ SWR meter/analyzer and the SWR is flat from 50Mhz to
>440Mhz.  Obviously there are a couple of peaks and dips.  But it has decent
>SWR in the ham bands.  It even has a F/B of approximately 15-20db based on
>S-meter.  Looking to run approximately 100 watts.

How wide band does your transformer have to be?  Only the 6m band?, or are
you trying to cover 2m and 220 and 70 cm too?

if narrow band...

What about one of those 4:1 transformers using a half wavelength of
coax?  That would get you to 200 ohms.

Or, do the 1/12th wavelength thing, 50 ohms any length, lambda/12 of 300,
lambda/12 of 50 ohm, any length of 300.

For a broad band transition, something like a taper transition might work
well.  Getting 50 ohms from an balanced openwire line is a bit tricky, but

Or, an open wire taper transition.  Z=120 invcosh(S/d). (S is center to
center spacing, d is wire diameter)  Inverting: S/d = cosh(Z/120)

For 50 ohms, S/d is 1.08 which is pretty darn close together. For 300 ohms,
S/d is 6.13

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