Topband: Open Wire Lines - Fact and Fiction

[K8LV1 at aol.com]

While recently reviewing some recent postings RE: open wire transmission 
lines, I found some controversial info to which I would like to add my 2 cents 
worth. Particularly intersting was the claim by W8JI (08/034/04) that he had 
built a perfect transmission line (almost). This triggered one of my alarms, the 
one that trips on statements that approximate "If it is too good to be true, it 
probably isn't". The data claims a 2-wire line, almost a third of a mile 
long+broadband transformers at both ends,  with a total attenuation of .087dB 
(2%), or .005db/100'. This data contradicts enough established RF engineering 
dogma that I simply can't believe it, and I believe that his measurments are 
flawed.The following three points support my opinion.

First, the line was described as 1500 feet long, #8 copper with 2 inch 
spacing, with details of the construction omitted. Assuming a straightforward 
construction, my calculations, based upon standard EM principles, shows that the 
attenuation constant at 4MHZ for an IDEAL line (no loss in the spacers or the 
medium) would be .00012 nepers/m. (RF units) or .03 db/100feet (ham units.) For 
1500 feet, this works out to .46db of attenuation (about 11% loss). Again, this 
is a totally optimistic calculation which assumes absolutely NO losses of any 
kind other than the .1ohm/m. of AC ohmic resistance at 4MHZ (properly 
corrected for skin-depth). These numbers are chiseled in stone - pure, basic, 
electronics.  

Second, the impedance of the line calculates to 412 ohms, which requires the 
noted transformers to get it down to 50 ohms. Broadband transformers, as 
stated in the data, would have to be constructed using ferrite cores. It is not 
possible to build such transformers, especially at transmitting power levels, 
which do not have losses that range from small but noticable, to overwhelming. If 
you doubt that, just spend about 10 minutes talking to anyone who has ever 
designed a broadband PA. The idea of building these matching transformers with 
power losses in the 1% range is just not thinkable. It would be possible to 
build transformers using low-loss coax, which would not be fully broadband, but 
would have losses that could be kept down to a few tenths of a db. over a 
single band.

Third, contrary to popular belief AND statements that I have seen posted 
here, even a perfectly 'balanced' open wire line will radiate some RF (an 
authoritative, early reference for this is the article by Sterba in Proc.IRE, 1932, 
p.1200) The statement that zero-radiation will occur is an approximation, that 
is useful when used properly. But if you want to engage in analysis of effects 
down in that 1% range, it won't work there. That is why the 4 wire 'X-line' 
was invented, to reduce the small but known amount of residual magnetic dipole 
radiation from the parallel line, which is really a long,skinny loop (i.e. a 
magnetic dipole). By using two equal and opposed lines, which is what the X-line 
really is, the magnetic dipole fields of each pair are highly cancelled, 
leaving a residual magnetic quadrupole field (whose radiation is orders of 
magnitude lower.) By reciprocity, it also greatly reduces the undesired signal 
capture in RX mode, which was one of the motivations for its invention.

An easier, intuitive way to fathom this phenomenon is as follows. Imagine 
taking that 1500 foot by 2 inch loop and stretching it out to a perfect square 
(which would be 750 feet on a side.) Such a big loop is obviously going to do 
some serious radiating at 4 MHz (or any other frequency for that matter). Now 
begin to pull the sides back in while lengthening it, gradually restoring it to 
the original line configuration. Is there any basic physical reason why at 
some "magic" value of the width of the loop, it should suddenly stop radiating? 
Of course not. The radiated field will just continue to decrease until the 
width shrinks to zero. 

Here is another interesting observation. When I went back and reviewed 
Sterba's article, there were many articles around that time on transmission line 
theory and measurements.The early radio antenna pioneers, like Brown, Kraus, 
Sterba, etc. were laying down the basic theory of HF radiating and transmission 
systems, which is all documented in the radio and electronics publications of 
the 30's and 40's I  noticed a general pattern in all of them, that the 
experimental measurements (of  line losses) were ALWAYS  higher by factors of 2-3 than 
the theoretical calculations based just upon the RF resistance of the 
conductors (as I used in my first point above.) Hence, for anyone wishing to engage 
in this type of loss estimation, I would suggest using the standard formulas 
for loss (based on pure ohmic loss) and then doubling the result.


In conclusion, I do not think an open wire (2) line with a loss of .005 
db/100'  is possible  in the HF ham bands. I would say that a long line like this 
would exhibit at least 1 db. total loss, from one 50ohm port to the other.


73
Eric von Valtier K8LV