Bob,
I believe everyone is talking about using two pieces of coax to simulate a
balanced at a length just long enough to get thru the wall of the house. So
by your numbers if this took 10ft of coax there would only be approx. 0.35
db of loss added to that of the ladder line.
Not enough to be concerned about.
However, I agree with you that running it all the way to the antenna,
using 100ft or more, is a bad idea. Not to mention it would be very heavy
and catch a lot of wind.
Dan
WD4LUR
At 05:39 PM 3/30/2000 -0600, you wrote:
>While using two pieces of coax in a physical configuration to simulate a
>balanced line may appear to be satisfactory, I point out that the loss
>in coax operating with a high SWR i.e. 10:1 is much much higher than
>open wire line with the same SWR figures. It would appear that using
>coax to obtain a balanced feed then defeats one of the purposes of a
>balanced antenna feed.......low loss.
>
>Typically 100' of RG-8 or RG-213 has 1.0 dB loss at 30MHz when matched.
>With a VSWR of 10:1 the additional loss is 2.5 dB so the overall loss is
>3.5 dB. For a 20:1 VSWR the additional loss goes up to 4.5 dB or an
>overall loss figure of 5.5 dB. To the contrary using 1" balanced wire
>line, the loss at 30MHz per 100 ft is 0.15 dB when matched. Thus with a
>10:1 VSWR the loss increases by 0.5 dB to produce an overall loss of
>0.65 dB and with a 20:1 VSWR the overall loss goes to 1.85 Db. Keep in
>mind that these numbers are based on impedance at the load or antenna.
>The VSWR as measured at the input or transmitter end of the transmission
>line will always be less due to loss in the transmission line. The more
>loss in the line the lower the VSWR will be at the source. Also, the
>more loss in the line, the less power delivered to the load.
>
>Keep in mind that a 3.0 dB line loss will reduce 100 watts by 50% or 50
>watts at the load and with 6.0 dB of line loss the 100 watts will be
>reduced by 75% thus yielding 25 watts at the load.
>
>73
>Bob K4TAX
>
>
>Jim FitzSimons wrote:
> >
> > Sherrill, my balun is made with a T-200-2 powered iron toroid.
> > A T-200-2 toroid is not small and it is not ferrite.
> > The band width on my balun is not as wide as a ferrite
> > balun but it will never saturate.
> > Large powdered iron toroids are higher Q than air wound
> > coils of the same size except at VHF frequencies.
> > The balun needs to be protected from water and ice like
> > any coil or connector. Powdered iron toroids do not absorb
> > water.
> > My house is made of metal and glass. It is a modern design
> > and it would not look good to have a ladder line coming
> > through the glass. I can hide the coax. The house makes
> > a great counterpoise.
> > Best regards, Jim W7ANF
> >
> > Sherrill WATKINS wrote:
> > >
> > > Carl: Very good question about the feasibility of bringing balanced
> line into the shack. I am aware of two methods to do this.
> > > The first and easiest method is to use two pieces of equal length
> coaxial cable, in parallel. Just tie the shields together at each
> > > end. The two center conductors form a shielded balanced line. ( This
> can be extended to the center of the dipole for a sheilded
> > > balanced line.) However, in most cases this is not necessary). The
> twin shielded balanced line will have a nominal impedance of
> > > twice the impedance of the coax; i.e. two lengths of RG-11 (75 ohms)
> connected in parallel will give a 150 ohm balanced line. Also,
> > > this is can be connected directly to 600 or 450 ohm ladder line with
> no problems. The two lengths of coax can be "snaked" through
> > > wall sections, etc. just like a single length of coax. The second
> and more "graphic" method is to locate the rig with the antenna
> > > tuner in front of a convenient window. Remove the glass window pane an!
> > > d replace it with clear acrylic or equivalent plastic. (This will
> require new window putty and possible paint to install the
> > > plastic. ) The plastic should be the same thickness as the glass so
> it will fit right in the sash.) Then drill holes in the plasic
> > > acrylic window pane and install grommits in the holes to fit the wire
> or coax size. Then just run the open wire line (or twin coax)
> > > through the grommits in the window!! I did this with open wire line
> through both a double sash window with an exterior aluminum
> > > storm window and it worked great!! (I did replace the storm window
> glass with acrylic sheet). The only draw back(?) is that this
> > > arrangement made my shack look like a shipboard radio room from the
> 1930's! (This was amusingly intimidating to visiting CB
> > > operators!) It also interfears with opening the window. Also,
> contrary to what most people have the mistaken opinon, if installed
> > > properly, open wire line does NOT radiate. It only will radiate if
> one has an i!
> > > mbalance in the antenna, i.e. one side of the antenna longer than
> > > radiating many years ago when hams connected one side of it to the
> end of a wire zep antenna, instead of the center, thereby creating
> > > a gross imbalance.) Also, when I was using my open wire system
> brought into the house, my tv set was about 15 feet from the rig. The
> > > tv was connected to 450 ohm tv twin lead to an outside antenna. I
> had no problems with tvi from my Drake TR4C running 300 watts.
> > > Finally, it is worth stating that I believe it is a mistake to use a
> small, cheap, ferrite type balun in an open wire system. This
> > > has been discussed several times in QST. The fact that one is
> mistakenly assured by a low vswr reading may be an indication that the
> > > balun is acting like a load resistor and converting some of the
> transmitted R.F. into heat. This gives a false sense of security
> > > because the swr meter is reading low. Also, I believe it is a
> mistake to mount any balum outside because of the effects of moisture
> > > will cause the balun to also start acting like a dummy load. - !
> > > Sherrill W. k4own.
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