You're correct that CB whips, 2 meter whips, AM broadcast towers, and
the like are just fine because of their large diameter.

But think about it a little bit. If you reduce the frequency by a factor
of two, the length of a resonant antenna increases by a factor of two.
The skin depth decreases by a factor of the square root of two, so the
resistance per unit length increases by this factor. So at half the
frequency, the resistance of a resonant antenna has increased by the
square root of two. Furthermore, you don't typically use 1/4 inch
diameter wire for an HF dipole, for example. (And most hams presumably
wouldn't make an HF dipole from surplus broadcast towers -- but then I
haven't seen your station.) Assuming you're using a resonant antenna of
constant diameter, the loss gets higher as the frequency gets lower.

Now go back and read what I wrote. Notice that I said that the wire was
most likely to be a problem with antennas on the order of an 80 meter
dipole length. Now you understand (I hope) why that qualification.

If you use an antenna at a frequency higher than its primary resonance
(such as a 40 meter dipole on 15, or a G5RV on a high frequency band),
the loss gets greater yet, since the skin depth decreases with frequency.

Roy Lewallen, W7EL

Richard Harrison wrote:
Roy, W7EL wrote:
"Some stainless steels are ferromagnetic, with a permability I`d guess
at 100 or more. That reduces the skin depth, and therefore increases the
RF resistance by a factor of 10."

Being ferromagnetic does not disqualify an antenna material. Most CB
whips are stainless steel. They aren`t too lossy because they are only
about 1/4-wave at 27 MHz and must be large enough in diameter to be
durable. In the range of 0.55 MHz to 1.7 MHz, most transmission is from
ordinary non-stainless steel towers. The only coating on many of these
is paint. The cross-section to length ratio is economically small, but I
doubt the loss added by using nonplated steel could be measured.

Best regards, Richard Harrison, KB5WZI

Roy, W7EL wrote:
"Notice that I said that the wire was most likely to be a problem with
antennas on the order of an 80 meter dipole length."

80 meters isn`t the lowest nor the highest frequency ham band, but could
be a problem for various reasons. Loss is directly proportional to
resistance in the familiar current squared times resistance formula.

Resistance increases due to skin effect only as the square root of the
frequency while resistance varies directly with the conductor length
which varies inversely with frequency.

Loss gets worse for a given wire size as frequency drops because you
need longer wire for a 1/4 wavelength. It doesn`t get worse as fast as
it would if there were no skin effect because as frequency lowers,
current penetration grows and wire resistance drops due to reduction of
the skin effect. But, the longer wire is inevitable along with its
higher resistance at the lower frequency.

At RFE, as soon as a new frequency became available we would often erect
a Signal Corps rhombic kit and start broadcasting. These weren`t made
for 100KW so the dissipation lines soon melted and the rhombics became
bidirectional.

At the end of WW-2, many Central Europeans fled to South America. When
we strengthened and replaced our dissipation lines, we discovered we had
built a large loyal audience among the fugitives. Not too surprising
since we were programming with some of their favorite entertainers who
were also exiles. Too bad we had to eliminate the signal flooding South
America, but other broadcasters had claims on the frequencies we were
using for a South American audience.

Best regards, Richard Harrison, KB5WZI

On Mon, 30 Aug 2004 10:36:57 -0500, (Richard Harrison)
wrote:

Roy, W7EL wrote:
"Notice that I said that the wire was most likely to be a problem with
antennas on the order of an 80 meter dipole length."

80 meters isn`t the lowest nor the highest frequency ham band, but could
be a problem for various reasons. Loss is directly proportional to
resistance in the familiar current squared times resistance formula.

Resistance increases due to skin effect only as the square root of the
frequency while resistance varies directly with the conductor length
which varies inversely with frequency.

Loss gets worse for a given wire size as frequency drops because you
need longer wire for a 1/4 wavelength. It doesn`t get worse as fast as
it would if there were no skin effect because as frequency lowers,
current penetration grows and wire resistance drops due to reduction of
the skin effect. But, the longer wire is inevitable along with its
higher resistance at the lower frequency.

At RFE, as soon as a new frequency became available we would often erect
a Signal Corps rhombic kit and start broadcasting. These weren`t made
for 100KW so the dissipation lines soon melted and the rhombics became
bidirectional.

At the end of WW-2, many Central Europeans fled to South America. When
we strengthened and replaced our dissipation lines, we discovered we had
built a large loyal audience among the fugitives. Not too surprising
since we were programming with some of their favorite entertainers who
were also exiles. Too bad we had to eliminate the signal flooding South
America, but other broadcasters had claims on the frequencies we were
using for a South American audience.

Best regards, Richard Harrison, KB5WZI

Very interesting, Richard. Were the rhombic dissipation lines made of high
resistance wire? And where were the transmitters located that flooded SA? And
what SA countries did the fugitives concentrate in?

Walt