There are several ways to use 75 Ohm CATV Hardline in 50 ohm systems.
The simplest method is to use a multiple of 1/2 Wavelength (electrical)
since impedances repeat every 1/2 WL in a transmission line. This works
well on narrow bands (15, 20M)) where the bandwidth is less than 3% of
the center frequency. I use this approach on all (5) of my monobanders.
On 10M, the bandwidth for DX (28000 to 29.1) is 4%. Same 4% BW
is needed for 40M (7000 to 7300). If the line length is 6 wavelengths
(~180 ft) there will be an electrical difference of 1/4 WL between 28.0
and 29.1 meaning that a 50 ohm load will present 50 ohms at the other
end of the feedline if tuned for 28.0 BUT will transform to 100 ohms at
29.1.
A good Broadband SINGLE BAND solution is to use an 'inline' transformer
made of 0.08 WL of 75 ohm coax followed by 0.08 WL of 50 ohm coax
to ANY length of 75 ohm CATV Hardline. One 'in line' transformer will
be needed at BOTH ends of the hardline. The 50 ohm line is connected
to the hardline at each end.
A good Broadband MULTI-BAND solution is to use toroidal transformers
designed to transform 50 ohms to 75 ohms at each end of the CATV
Hardline. Any length of hardline may be used with this solution.
Tom N4KG
On Thu, 18 Jul 2002 "Troy Flowers" <troy.flowers@wavecom-inc.com> writes:
> Ok I'm a relative newbie so I'll ask also. I ran 1/2" Heliax up my
> tower for HF. But 75 ohm CATV line can be used also? Being a
cheapskate
> I'd much rather spend nothing so explain. My radio/amp wants 50 ohm
but the
> feed line is 75 ohm and the antenna is some range also based upon freq.
> Are you> supposed to do a complex analysis of this or is it safe to
take a
> simplistic view of it (in terms of station performance)?
> TIA
> de KE6ASO
> Troy also.
>
> -----Original Message-----
> From: towertalk-admin@contesting.com
> [mailto:towertalk-admin@contesting.com]On Behalf Of Pete Smith
> Sent: Thursday, July 18, 2002 11:01 AM
> To: Red
> Cc: 'Troy Thoele'; TOWERTALK@contesting.com
> Subject: Re: [Towertalk] Newbie Question
>
>
> At 12:44 PM 7/18/02 -0500, Red wrote:
> >Troy;
> >
> >Normally I'd agree that heliax is overkill on HF, but suggest you
> check
> >the attenuation of any coax, expecially on 10 and 15 meter.
> Ordinary RG-8
> >loses about 1.2-1.3 dB per 100 feet at 30 MHz. That amounts to 3 -
> 3.25
> >dB for 350 feet; half the power!
>
> But 75-ohm 3/4" CATV hardline is just a hair lossier than 7/8"
> Heliax at 30
> MHz (.3-.35 dB/100 ft), can often be bought for free, and lends
> itself to
> inexpensive connector fabrication.
>
>
> 73, Pete N4ZR
>
> Check out the World HF
> Contest Station Database at
> www.pvrc.org
>
>
>
>
> _______________________________________________
> Self Supporting Towers, Wireless Weather Stations, see web site:
> http://www.mscomputer.com
> Call 888-333-9041 to place your order, mention you saw this ad and
> take an
> additional 5 percent off
> any weather station price.
> _______________________________________________
> Towertalk mailing list
> Towertalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
> _______________________________________________
> Self Supporting Towers, Wireless Weather Stations, see web site:
> http://www.mscomputer.com
> Call 888-333-9041 to place your order, mention you saw this ad and
> take an additional 5 percent off
> any weather station price.
> _______________________________________________
> Towertalk mailing list
> Towertalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
________________________________________________________________
GET INTERNET ACCESS FROM JUNO!
Juno offers FREE or PREMIUM Internet access for less!
Join Juno today! For your FREE software, visit:
http://dl.www.juno.com/get/web/.
|