[AMRadio] new antenna


D. Chester k4kyv at charter.net
Mon Mar 12 12:00:26 EST 2007


The tolerable level of SWR depends on  the feedline used.  With ordinary 
coax, I would try to avoid anything over 3:1.  The problem caused by high 
SWR is dielectric loss, plus the possibility of exceeding the rf voltage 
reading at high voltage points, and overheating marginally sized cable at 
high current points.  With open wire line, a SWR of 10:1 or more is not 
unusual, and if the wire size is large enough and the spacing between wires 
is wide enough, there is no problem.  The feedline loss of open  wire tuned 
feeders is very low even at high SWR.

By definition, a "tuned" feedline is a feedline with substantial SWR.  A 
"flat" feedline theoretically has a 1:1 SWR.  It's a matter of properly 
designing the antenna/ feedline system.  Coax is too lossy to use as a tuned 
feeder, but the loss in open wire is usually not a problem, even at  high 
SWR.  But there is nothing magic about running a flat feedline.  As long as 
the current and voltage limits of the coax are not being exceeded, anything 
below 2.5:1 or 3:1 should be ok.  But at those extreme limits of SWR, the 
coax exhibits some aspects of a tuned feeder system.  The impedance the 
transmitter sees into the feedline will vary according to the length of the 
feeder. That explains why sometimes the performance of a coax-fed antenna 
may vary depending on the length of the coax feedline.

A "transmatch" is used between the transmitter and coax feedline to 
transform the impedance seen at the transmitter end of the feedline to 
something within the range the transmitter is designed to work into.  In 
amateur work, modern solid state rigs typically are designed to work into 
50-ohms, non-reactive, and tolerate very little variation from that figure 
of load impedance.  According to the manual, the Gates BC1-T and later ones 
in the Gates series of 1 kw broadcast transmitters, are designed to work 
into a 50 to 70-ohm  load.  With 2.5:1 SWR on the coax feedline running out 
to the base of the tower, I could not get mine to load up to full power at 
the extreme edges of 160, so I designed and built a simple variable 
L-network to use as a transmatch between the transmitter and coax feedline.

Open wire tuned feeders are a different matter.  An "antenna tuner" is 
required.  This matching network usually  has to more radically transform 
the impedances than would a simple transmatch.  The best antenna tuner 
circuit is the classic link-coupled balanced tuner with split stator tuning 
capacitor.  The tuned circuit may be wired as series resonant to feed a line 
that presents a low impedance to the matching network, or wired in parallel 
to present a high impedance.  Intermediate impedances may be matched by 
tapping the feeders down on the coil in the parallel tuned configuration, 
although this can be an "iffy" proposition.  Better to use the appropriate 
lenghth of feeder to present a high voltage point (high impedance) or a 
high current point (low impedance) to the matching network.  The worst 
possible circuit to use as an antenna tuner to feed open wire balanced line 
is what most commercial tuners use:  an unbalanced L- or T- network, working 
into a bal-un on the feedline side of the tuner.  Baluns are not designed to 
work into highly reactive  loads, and usually are designed to work into a 
very limited impedance range.  The highly  reactive load, at widely varying 
impedance, that appears at the transmitter end of a typical amateur 
open-wire tuned feeder installation, is inevitably far beyond the range of 
the balun at certain frequencies on certain amateur bands.  This results in 
excessive RF loss and possible overheating of the balun, especially if it 
uses a ferrite core.  I have heard of ferrite core balun coils literally 
exploding like firecrackers at high power.  In addition, the ferrite core in 
a balun may be overdriven into saturation at the peaks of the RF sinewave, 
distorting the waveform, and causing the radiation of RF harmonics, which 
could result in interference to non-amateur services (including, but not 
limited to TVI), and possibly an FCC citation.

If the open wire feeder is properly balanced, there should be no feedline 
radiation.  Unbalance in the current in the feeders indicates COMMON MODE 
current in the feedline.  Per the laws of physics, the rf feeder current 
must be balanced, just as both wires of a two-conductor cable feeding a 
resistor must carry the same current when DC is applied from a battery.  But 
in addition to the RF feeder current, the feeders may also carry "antenna 
current"  (common mode) which means that the entire feedline is acting as a 
single conductor.  Superimposed on the feeder current, this will appear as 
an unbalance between the RF current on one feeder vs current on the other 
one.  This common mode, or antenna current on the feeder will radiate when 
transmitting, or pick up signal when receiving.  With coax, common mode 
current will appear as a current on the outside of the coax braid, and will 
radiate just as would a solid piece of wire carrying the same current.  If 
there is no common mode current, the balanced current will reside on the 
centre conductor of the coax and the inside of the braid, and there will be 
no radiation.  With open wire balanced line, if there is no common mode 
current, the currents in the feeders will exactly balance, and there will be 
NEGLIGIBLE radiation.  That means a tiny fraction of the energy will be 
radiated because, with open wire line, both conductors cannot occupy 
precisely the same space, and thus the open feeders act like a two-element 
antenna, but with the elements so closely spaced that the radiation of one 
virtually cancels out the radiation of the other.

As you go higher in frequency, there will be more and more radiation from 
balanced open wire feeders.  Try using 6" spaced 600-ohm open wire line, 
very commonly used at 1.8 - 30 mHz, at 300 mHz, and you will have very 
substantial  radiation.  That same feedline when used on 160m will have 
essentially zero radiation if the feeders are closely balanced (carrying no 
"antenna," or common mode current).

With coax, a choke balun near the feedpoint of a centre-fed dipole may 
eliminate the common mode current.  There will inevitably be at least a 
small amount of common mode current when a coax line directly feeds a 
balanced dipole, since at the feedpoint of the dipole, the RF voltage at 
each leg cannot be zero (if it were zero that would mean zero energy was 
being fed into the antenna), so therefore, the coax braid cannot be at zero 
RF voltage where it feeds the antenna.  This common mode rf voltage will 
form standing waves along the coax braid, just as it would on a single wire 
antenna.  That is why the common mode current is sometimes called "antenna 
current."  It is current that makes the feedline act like an antenna ard 
radiate.

With open wire feeders, it's a matter of physical balance, such as getting 
the wire lengths exactly the same on both sides of the dipole, making sure 
the two conductors making up the line are the same length, keeping both legs 
of the dipole  the same distance above the ground and the same distance away 
from large metal objects and other feedlines and antennas. 




More information about the AMRadio mailing list

This page last updated 18 Oct 2017.