The practice of making a feedline a multiple of the working wavelength has
been around for a long time. Lots of arguements have been heard on both
sides of the practice. It is usually agreed that it is less important
when the length of the feedline is long in relation to the operating
frequency where the match is between the transmitter and the feedline and
the antenna and the feedline. Having said that, if you wish to follow the
practice, you must take into account the "velocity factor" of the feedline
you are using. The formula that you quote is generally used for determing
the length of an antenna itself and is based on a velocity factor of .95
which is about right for an 18 ga wire in free space (actual factor for
a half wavelength is 492 for rf in free space).
Since the velocity factor for coax is much less than .95 (usually around
.75) you must adjust for it if you want your coax to be a multiple of the
wavelength. Find the velocity factor for the coax you wish to use and
then use 492 * K / frequency in MHz for the proper length in ft. ..K being
the velocity factor for your coax.
Rich Gort (W7JVL)
WWP19 #202 SAYOKO (Oct, 83)
Lake Stevens, WA
Sailing Northern Puget Sound and North Cascades Mountain Lakes
On Fri, 10 Dec 1999, Bill Blohm wrote:
> Good going. Something else that could help, at least when I first
> started messing with antennae this is what I was told, is to use
> a length of cable that's a multiple of the desired main operating
> frequency. That is, take the wavelength of the desired frequency
> and make the cable length some multiple of that. Let's say your
> frequency is 144.39 MHz. Your wavelength is 6.48'. Say your length
> from antenna connector to radio connector is 33 feet. Since 5
> wavelengths is only 32.4', you either re-route to use 32.4' or go
> with 6 wavelengths, or 38.88' of cable and lay the cable to neatly
> hide the rest of the cable. You can, of course, cut the cable to
> run 33', but there will be some slight mismatch.
>
> The way I learned it was .5L = 468/f
>
> where L is wavelength and f is the frequency in megahertz. Note
> that this provides you with half the wavelength, not the full
> wavelength.
>
> Bill B.
> P-19 #454, Dream Catcher
> Nampa, ID
>
> P.S. 144.39, for those of you that might be curious, is the
> national APRS frequency.
>
> Jim Nolan wrote:
> >
> > - - - - - - - - - - - - - - - - - - - - - - - - - - -
> > West Wight Potter Website at URL
> > http://www.lesbois.com/wwpotter/
> > - - - - - - - - - - - - - - - - - - - - - - - - - - -
> >
> > >From: Rye Gewalt <ryeg@vais.net>
> > >To: charles@falk.net, Potter List <wwpotter@tscnet.com>
> > >Subject: Re: Coax Connectors
> > >Date: Fri, 10 Dec 1999 05:33:57 -0500
> > >
> > >Charles Falk wrote:
> > >
> > > > Would a couple of BNC or TNC connectors on RG58 pigtails be better than
> > > > a PL259 on the cable from the mast connecting to an SO239 bulkhead
> > > > connector on the cabin roof?
> >
> > The UHF connectors can handle alot more power than either BNC or TNC.
> > UHF~2kW, BNC~10W TNC~25W. Also the UHF is less bothered by corrosion because
> > of its large area. A UHF connector that can feed through the cabin roof is
> > available from Pasternack Enterprises 949-261-1920, Irvine, CA. It is part
> > #PE9114 UHF female to UHF female bulkhead mount. It costs $8.95 in single
> > piece quantities. I would cover the outer portion with a rubber cap when it
> > is not in use.
> >
> > Do not splice coax cable. You can get away with splices in short runs (less
> > than 12" at 150 MHz) but on the boat the cable is 20-50 ft long. Any splice
> > or impedance bump will seriously limit the match between the antenna and
> > radio and could in some cases burn out the radios final amplifier. Use
> > connectors to join cables.
> >
> > Also a better cable than RG58 and still the same size is RG142. It has half
> > the loss but costs about 4 times as much. RG58 will have about 3 db loss per
> > 100' at 150 MHz and RG142 will have about 1 dB. In a typical boat
> > installation you will get about 60% more power to the antenna using RG142
> > over RG58. RG58 is .40/ft in 100' qty, RG142 is $1.63/ft in 100' qty.
> >
> > Over and out,
> >
> > Jim Nolan P-19 #426 Panache
>
> > Get Your Private, Free Email at http://www.hotmail.com
>