On the subject let me recount a tale from days gone by!
Many years ago when I lived in the UK, I ordered 4 antennas from Jaybeam
for 432 MHz. I arranged to drive down to Northampton to collect them as I
was also going on to London for another reason. In those days I lived in
Liverpool. On arriving at Jaybeam I was greeted by one of the salesmen and
taken into the stock room which was lined from floor to ceiling with row
upon row of dexion racking containing finished antennas packed in cardboard
boxes. The label on the end of the box had the type number and a "50 / 75
ohm" label.
The salesman asked me what impedance I wanted as the antennas were
available in either 50 or 75 ohm versions. When I told him I wanted the 75
ohm version as I was going to feed them with CATV hardline, he took a large
felt-tip pen and crossed out the 50 ohm label on the end of the boxes!
I asked him why he had done that and he replied "Well you see the antennas
are actually designed for 60 ohms and so they work equally well on 50 or 75
ohms".
Later I discovered that in Germany the standard impedance for many years
(maybe still is?) was 60 ohms and I saw some old test equipment at a German
amateur flea-market which had 60 ohm Dezifix connectors.
There may be a moral to this story, design your antennas for 60 ohm
impedance!
Incidentally the 75 ohm CATV cable is in my opinion the best for the price,
75 ohms has the lowest loss per unit length, whereas 50 ohm cable came
about as a quirk of chance during WW2 as a convenient way of making low
loss solid transmission line using stock size copper tube. I read an
interesting article in an old American magazine as to how it came about,
maybe someone has a copy lying around?
John ZS5JF
----------
> From: David A. Pruett <k8cc@ix.netcom.com>
> To: amps@contesting.com
> Subject: [AMPS] 50:75 ohm matching
To: <amps@contesting.com>
> Date: 24 December 1998 06:03
>
>
> Carl, KM1H wrote:
>
> >Why on earth do that? Any rig or amp worth using will load directly into
> >75 Ohms with no problem. Not all the world has 50 Ohms as a standard and
> >the mfg designs for the variations.
>
> Up until I installed my 80M four-square using a ComTek switchbox, I would
> have agreed. My entire station (four towers and all monoband antennas)
> uses
> CATV hardline. For most bands I use asynchronous transformers made of 50
> and 75 ohm cable to calm any mismatch concerns. With the 80 four-square,
> I figured "why bother - its eighty meters, right?" Well, with a properly
> installed array centered on 3650 at the end of approximately 270' of 1/2"
> CATV hardline, the SWR at the shack end was near 1:1 on in the phone DX
> band
> and 2:1 on CW. At the load end it was near 1:1 in both places.
>
> I suspect that the length of the feedline was approaching being a
quarter-
> wave multiple down near the CW end although I did not confirm this.
> Installation of a pair of homebrew 50:75 toroidial transformers from N8CC
> brought the SWR to near unity on both ends of the band.
>
> (Note: for those who are not familiar with the ComTek box, it uses
quadra-
> ture feed and a dump load to drive the array. While the resulting SWR on
> the feedline is near unity, the normal SWR variations vs. frequency are
> masked by the drive system. I'm NOT claiming that the 50:75 transformers
> made the antenna broadbanded.)
>
> In my case, the 50:75 transformers allowed the system to function
correctly
> rather than forcing me to "tune" the feedline (a task with somewhat
limited
> flexibility seeing that the feedline is buried).
>
> 73,
>
> Dave/K8CC
>
>
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