[Antennas] Another dipole swr Question

2002年6月14日 16:15:54 -0500

John, you haven't said, but I get the idea that you are not using a tuner
with your dipole on 75. I think that you will find that the SWR bandwidth
that you get is mainly a function of the antenna and frequency and not of
the feedline, etc.
Of course, if you can start with a lower minimum SWR then the SWR at the
edges will be lower. How much I could not say without modelling the system,
but my gut feel is "not much." I think that you would probably do better on
SWR bandwidth if you would put up another flat top wire in addition to your
present antenna. Make it a little longer or shorter as needed to extend the
SWR bandwidth in the desired direction. Separation of the two antenna wires
will help a little to reduce interaction. 
Parallel-fed dipoles like this are probably the quickest and easiest way to
buy some bandwidth. Take that back, the easiest way is to just put a
resistor across the feedpoint! Then you would have an antenna as good or
better than the Double Bazooka or the B&W jobbie.
But, what you have already is pretty good, as we will see . . .
First, though, I doubt that you will find much significant improvement by
adding the series section. If the antenna were around 75 ohms then you
would need a 1/4-wave section of 62 ohm coax for the match ( sq root of 50
x 75). A 75-ohm section is the matching impedance between 50 ohms and 112.5
ohms.
One approach might be to just run 75 ohm coax from the antenna to the shack
and use a small tuner to handle the excursions and make the 75 to 50 ohm
transformation in the shack. I find it much more convenient to twist knobs
and switches than to prune coax, etc. And that series section does almost
nothing to extend SWR bandwidth - it just moves the curve down a little.
Let's look at the numbers to see what would happen on 75 meters, using 100
feet of RG-213, if you just fed your present antenna with its assumed 75
ohm resonant input resistance.
You would lose less than 0.4 dB/100 ft if the line were matched. Connected
to a 75 ohm load your transmitter sees an impedance of about 58 ohms at an
SWR of about 1.45 at 3.85 Mhz. The 1.45 comes from the fact that the
magnitude of the line input impedance is abaout 58 ohms, but it is a
complex impedance and the reactive component raises the SWR.
If the SWR went up to 5:1 at the band edges, and you used a tuner to
present the transmitter with a 50 ohm resistive load, the added loss in the
coax would be 0.45 dB for a total line loss of less than one dB. Those
figures are for 100 ft of feedline - a shorter line would should less loss,
of course.
It is interesting to look at how the loss is distributed at 3.85 MHz, if
that is your resonant (minimum SWR) frequency. For 100 watts input, about 8
watts will be lost in the line whether it is matched or not. The mismatch
to 75 ohms at the center frequency only adds 0.6 watts of loss to the
system for a total loss of under 9 watts, including a very small dielectric
loss of under 0.2 watt. If the SWR went up to 5:1 at the band edges the
line loss would increase by only 0.45 db. Altogether the line loss over the
entire band does not represent a dB change that anyone on "the other end"
could detect.
I don't know about you, but I would leave the antenna and present RG-213
feedline alone, use a tuner to handle the line input impedance variations
and enjoy operation from 3.5 to 4.0 MHz with a total line loss of less than
one dB that no one, including you, will ever notice. 
There is a lot of misinformation around on 75/80 meter antennas and many
arcane approaches have been taken to get a "flat line" over the whole band.
As it happens (see the numbers above) the consequence of NOT having a flat
line is to lose a negligible amount of power as well as the convenience of
operating anywhere in the band with only a simple dipole in the air,
instead of some coax monstrosity or a huge cage dipole or the like. Keep in
mind that a "flat line" does nothing to help the antenna radiate. In
theory, it gets a miniscule amount of additional power to the antenna, but
using the numbers above, we see that "miniscule" is small indeed.
I operate 20 meter Extended Double Zepps on 80 with ladderline SWR as high
as 12:1 without any noticeable ill effects. Line loss is almost
unmeasureable. And I can operate any frequency from 3.5 to 29.7 MHz . . .
Try it!
73/72/oo, George W5YR - the Yellow Rose of Texas 
Fairview, TX 30 mi NE of Dallas in Collin county EM13qe 
Amateur Radio W5YR, in the 56th year and it just keeps getting better!
QRP-L 1373 NETXQRP 6 SOC 262 COG 8 FPQRP 404 TEN-X 11771 I-LINK 11735
Icom IC-756PRO #02121 Kachina 505 DSP #91900556 Icom IC-765 #02437

John wrote:
>> Thanks for all the good info. I know the swr of 1.6 is no problem, But my
> concern is the bandwidth on 75 meters. With the swr starting at 1.6 you lose
> a lot of bandwidth. I would like to use a 75 ohm coax quarter wave matching
> section to get the swr lowest as possible from the start........ Will this
> work ??