January 1965 Electronics World
Wax nostalgic about and learn from the history of early electronics. See articles
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
The last time I remember using 300-ohm twin-lead transmission
cable for a television antenna is about 20 years ago when I
lived in a fairly rural area and had a VHF/UHF aerial mounted
on a 20-foot tower with a rotator. Since then the connections
have always been 75-ohm coax either to wired cable or a satellite
dish (now I have nothing). I do,
however still have 300-ohm twin-lead running from my ½-wave
dipole antenna that is used for FM radio reception. Most people
more than 40 years old remember seeing the old twin lead running
down across the house roof, down the side, and through the living
room wall. My guess is that little consideration was given to
cable routing and how it might affect the performance of the
cable. Twin-lead cable is amazingly low loss - typically much
lower than coaxial cable - when properly suspended. Its weakness
is its vulnerability having its impedance altered by nearby
structures. Coaxial cable largely solves the proximity problem
because in an ideal coaxial cable, all the RF current flows
on the outer surface of the center conductor and on the inner
surface of the outer shield. Author Mark Nelson conducted a
simple experiment to determine how twin-lead cable routing affects
line loss. Impressively, twin-lead cable is fairly tolerant
of non-ideal routing as long as it remains dry and does not
lie close to ground.
Loss Figures for 300-Ohm Twin-Lead
By Mark L. Nelson
Catalogues usually give the characteristics of 300-ohm transmission
lines in free space. These figures will change considerably
when used in actual installations.
Almost every TV antenna in the country uses 300-ohm twin-lead
for the transmission line. These transmission lines are installed
with procedures that time and experience have shown to be best.
Other than experience, however, what do we really know about
300-ohm lines? What effect do water, metal, or close proximity
to wood or the earth itself have on 300-ohm transmission lines?
The only published figure the author found stated that the
300-ohm flat transmission line has a loss of 1.7 db per 100
feet at 200 mc. (Ch. 11 = 198-204 mc.). Taken quite literally,
this means that a 200-mc. signal would travel through 353 feet
of 300-ohm twin-lead before losing 6 db, or 1/2 of its original
voltage. This is quite an achievement for such a low-cost transmission
line. For all practical purposes, though, this figure is useless
because it is a computed, free-space loss that is not valid
in actual practice.
Lacking any further information, it was decided to find out
exactly how good 300-ohm twin-lead is and the extent to which
various practices affect its transmission qualities.
Fig. 1 shows the test setup used to sweep the twin-lead over
the TV frequencies of interest (54-216 mc.)
Fig. 1. Test setup for determining the characteristics
of typical 300-ohm transmission line under various conditions.
Sweep generator "H" output drives scope horizontal circuit.
The loss of the line at different frequencies, and under
different conditions, is determined by adjusting the variable
attenuator until the detected outputs of path A and B are equal
as observed on the scope.
Using the test setup shown, a group of tests was made to
determine loss vs frequency. The 300-ohm transmission line used
in these tests was 100 feet of Belden No. 8225.
Test 1. 100 feet of twin-lead was routed along a wooden wall,
simulating a normal installation with 3 1/2" screw insulators
every ten feet.
Test 2. 50 feet of twin-lead was supported with insulators
and 50 feet of twin-lead was stapled with Romex staples across
the line. Staples were placed five feet apart.
Test 3. 100 feet of twin-lead supported by wooden blocks
was placed just above ground level. Distance above ground was
three feet at wooden supports and two feet at mid-span between
Test 4. 100 feet of twin-lead was lying on the ground.
Test 5. 100 feet of twin-lead was installed as in Test 1
with the twin-lead running through a three-foot section of aluminum
Test 6. 20 feet of twin-lead was soaked in water for two
hours. The results were multiplied by five to obtain the figures
for 100 feet of line.
From the preceding experiments, the results shown in Fig.
2 were obtained. (The figures take into consideration the matching
losses from the matching transformers.)
Fig. 2. Results of six tests made on typical
Posted February 16, 2015