[Antennas] Balanced line optimum conductor spacing?
Gene Mason
[email protected]
Sat, 05 Jan 2002 04:14:29 +0000
<html><div style='background-color:'><DIV>
<P>Comment.. Is this a case of the 'proof is left up to the student' ? btw, the last time I checked, Maxwells equations didn't discriminate between positive and negative currents. </P>
<P> In a transmission line, frequency doesn't enter into the calculations, until the line losses become excessive at the upper frequency limit. The technical data on designing and constructing open wire feeders, suggests that the wire is not insulated and low loss spreaders are used. To do otherwise will produce undesireable results. At very high frequencies, there exist alternative transmission devices, these are called waveguide at microwave frequencies.</P>
<P> KZ5V Gene Mason<BR><BR></P></DIV>
<DIV></DIV>
<DIV></DIV>>From: Dave Shrader <[email protected]>
<DIV></DIV>>Reply-To: [email protected]
<DIV></DIV>>To: [email protected]
<DIV></DIV>>Subject: Re: [Antennas] Balanced line optimum conductor spacing?
<DIV></DIV>>Date: Fri, 04 Jan 2002 19:32:25 -0500
<DIV></DIV>>
<DIV></DIV>>The answer is really founded in Electromagnetic Field Physics. Each side
<DIV></DIV>>of the balanced line radiates. One line is radiating from a positive
<DIV></DIV>>current, for example, while the complimentary line is radiating based on
<DIV></DIV>>a negative current. At a 'far field' broadside pattern the vector sum of
<DIV></DIV>>the two fields cancel for a net of zero radiation. [Note: it's been too
<DIV></DIV>>many years from my academic days, 43 years, to do the math, but the
<DIV></DIV>>principle is well understood]. For broadside configurations the spacing
<DIV></DIV>>is secondary, but for collinear configurations the spacing in electrical
<DIV></DIV>>degrees becomes more critical. There will ALWAYS be some radiation in
<DIV></DIV>>the collinear configuration, but if it is small ... ???
<DIV></DIV>>
<DIV></DIV>>In reality, good engineering practice and compromise provides a workable
<DIV></DIV>>solution. A spacing of 10 degrees at the highest frequency of interest
<DIV></DIV>>will begin to produce noticeable changes in the antenna pattern. This is
<DIV></DIV>>a spacing of about 10 inches for 10 meters. The 10 inches is
<DIV></DIV>>approximately 720 ohms versus 4 inches at 600 ohms. A design of 3.6
<DIV></DIV>>degrees spacing at the highest frequency produces a 1% imbalance in the
<DIV></DIV>>antenna pattern.
<DIV></DIV>>
<DIV></DIV>>So, Radio Shack ladder line at approximately 1 inch spacing should be
<DIV></DIV>>usable up to 2.5 meters with negligible impact on antenna pattern, and
<DIV></DIV>>up to 2 meters with minor impact. While a 4 inch line has a 'max'
<DIV></DIV>>frequency of about 10 meters.
<DIV></DIV>>
<DIV></DIV>>Here, science [Physics] and engineering [Electromagnetic Design] will
<DIV></DIV>>never agree as to the degree of precision; but, engineers use the tools
<DIV></DIV>>of Physics to provide workable but not perfect solutions.
<DIV></DIV>>
<DIV></DIV>>With that caveat stated: I am a retired engineer, not a retired
<DIV></DIV>>Physicist!
<DIV></DIV>>
<DIV></DIV>>W1MCE
<DIV></DIV>>
<DIV></DIV>>Tom Scott wrote:
<DIV></DIV>> >
<DIV></DIV>> > All the discussion of window line (which I've always preferred) and ladder
<DIV></DIV>> > line, got me to wondering if anyone has seen any definitive tests or
<DIV></DIV>> > analysis of the effects of the spacing between the two balanced conductors?
<DIV></DIV>> >
<DIV></DIV>> > For a given frequency there should be a tradeoff between wider spacing that
<DIV></DIV>> > yields lower loss and higher impedance versus poorer balancing of the
<DIV></DIV>> > complementary fields surrounding the two conductors. I seem to remember
<DIV></DIV>> > reading somewhere that 0.1% of the wavelength was a good compromise, but I'm
<DIV></DIV>> > not certain. That would suggest that for the 10m band you should use 10mm
<DIV></DIV>> > wire spacing (~0.4"), and for the 160m band you should use 160mm wire
<DIV></DIV>> > spacing (~6.3"). It seems that most home-built ladder line I've seen is
<DIV></DIV>> > built with something on the order of five to six inches spacing, while
<DIV></DIV>> > window line is around 1 inch or a little less, so this calculation seems
<DIV></DIV>> > like it's in the ballpark. Since we hams have a nasty habit of wanting to
<DIV></DIV>> > operate on multiple bands, I'm just curious how much balance you lose with
<DIV></DIV>> > real wide conductors when operating from a higher frequency band, and how
<DIV></DIV>> > much extra loss you get when you operate narrow spaced lines on a lower
<DIV></DIV>> > frequency band.
<DIV></DIV>> >
<DIV></DIV>> > Probably not too significant, but just something that got me wondering.
<DIV></DIV>> >
<DIV></DIV>> > As I recall, the NEC2 models don't yield much useful information about
<DIV></DIV>> > balanced lines.
<DIV></DIV>> >
<DIV></DIV>> > Maybe one of the transmission line calculators could give the results of
<DIV></DIV>> > this analysis. Of course there are other factors, like conductor quality and
<DIV></DIV>> > diameter, dielectric quality, etc.
<DIV></DIV>> >
<DIV></DIV>> > - Tom Scott, Field Applications Engineering
<DIV></DIV>> > _/_/_/_/_/_/_/_/_/_/_/_/_/
<DIV></DIV>> > Arrow Electronics
<DIV></DIV>> > 9500 SW Nimbus #E
<DIV></DIV>> > Beaverton, OR 97008
<DIV></DIV>> > 503-614-1223 - Office
<DIV></DIV>> > 503-614-0123 - Branch
<DIV></DIV>> > 503-645-0611 - Fax
<DIV></DIV>> > 503-330-6867 - Cell
<DIV></DIV>> > 503-538-5839 - Home
<DIV></DIV>> > KD7DMH
<DIV></DIV>> > _/_/_/_/_/_/_/_/_/_/_/_/_/
<DIV></DIV>> > "The wireless telegraph is not difficult to understand.
<DIV></DIV>> > The ordinary wired telegraph is like a very long cat. You pull on the tail
<DIV></DIV>> > in New York and it meows in Los Angeles.
<DIV></DIV>> > The wireless is much the same, only without the cat."
<DIV></DIV>> > -- Albert Einstein
<DIV></DIV>> >
<DIV></DIV>> > _______________________________________________
<DIV></DIV>> > Antennas mailing list
<DIV></DIV>> > [email protected]
<DIV></DIV>> > http://mailman.qth.net/mailman/listinfo/antennas
<DIV></DIV>>_______________________________________________
<DIV></DIV>>Antennas mailing list
<DIV></DIV>>[email protected]
<DIV></DIV>>http://mailman.qth.net/mailman/listinfo/antennas
<DIV></DIV></div><br clear=all><hr>Send and receive Hotmail on your mobile device: <a href='http://go.msn.com/bql/hmtag2_etl_EN.asp'>Click Here</a><br></html>