[Antennas] SWR Isn't Enough

2002年4月28日 08:16:13 -0400

Well, I started this thread with the sole intention of answering a
question regarding a good 20, 17, 15, 12 and 10 meter 'beam' antenna for
under 600ドル. A Log Periodic fits the bill as a broadband driven array
with gain, ease of matching and available at reasonable price.
SWR is NOT a MEASURE for any antenna!! The current generation of HAMS is
VSWR paranoid!! Why, I run about 30:1 VSWR on 80/75 meters and I have an
excellent 80/75 meter signal!! I have a VSWR of about 10:1 on 30 meters
and have worked the South Pacific with great signals! My low band
antenna, 80/75, 40 and 30 meters, is a center fed doublet as a VEE, peak
at 35 feet, fed with 600 ohm open line and a good tuner. In the early
days of Ham Radio, I started in the early 1950s, no-one even measured
VSWR but we all had great signals.
During my days as a Aerospace Systems Engineer/Chief Engineer we would
boast that we can make any conductor or dielectric radiate!!
So, what makes a good antenna? We must begin by asking "What is a good
antenna?". I've designed antennas for the USAF with negative gain, -10
to -12 dBd, that totally satisfied my USAF customer. I've also been
responsible for producing +40 dBd gain antennas for USAF in a satellite
application. To do this requires an understanding of the system
communication requirements, i.e. specifications. As HAMS we fail to do
this. So we put up the best we can based on finances, restrictions,
other people's experiences, our own previous successes and failures,
frustration levels and finally the most important requirement "What will
my wife say about it?"
Some 3 element Yagis boast 6 dBd gain but construction methods yield an
antenna that is only 30 to 40% efficient! While others, single band Long
John's [5 or 6 elements on 50 foot booms], deliver 8 to 9 dBd gain, very
good efficiency [~90% or better] over a relatively narrow bandwidth. 
As an ARRL Registered Instructor I recommend that the 'new ham'
establish a station budget, example 1,000,ドル for the first station. Now
divide that budget into two unequal parts, approximately 1/3 and 2/3s.
Having done that, I next ask "Which part should be spent on the
radio/transceiver?" Most everybody fails!! 2/3rds should be spent on the
antenna!!!!
So, an antenna has to meet several sometimes conflicting requirements.
Within a budget, select Effective Aperture [gain], EFFICIENCY [as
important as gain], bandwidth, planned usage, planned installation
details, etc. If you intend to work only 20 meter DX/CW then a LP is not
your antenna, a monoband Long John tuned for 14025 kHz is the natural.
But, if you want 20, 17, 15, 12 and 10 meters PLUS MARS 13.5 to 30 MHz
capability then an LP is the natural choice.
Which would you choose? An antenna with 8 dBd gain at 50% efficiency.
Or, An antenna with 6 dBd gain and 99% efficiency. [Efficiency is the
ratio of power lost IN the antenna to power delivered to the antenna].
So, in my opinion, a good HAM antenna must be suitable to it's intended
usage as one component in a communications system. It should be
Efficient, competitive in gain, and meet your bandwidth requirements.
Having done all that you now need the face the issue on how to couple rf
energy into your dream antenna. This is where matching, VSWR, becomes a
system requirement. Your radio has a designed load impedance requirement
and your antenna may present a different load. You may want to use 50
ohm coax, 75 ohm coax, 300 ohm twinlead, 450 ohm ladder line or 600 ohm
open wire feeders. You may decide on single stub matching, double stub
matching, network at antenna, tuner in shack, or a 100% 50 ohm system.
VSWR is only a measure of the matching performance over a frequency
band. [This can be narrow band as in the case of a Long John, or wide
band as in the case of a LP].
In Conclusion, an HF beam type antenna should meet your requirements for
gain, efficiency, and bandwidth. Only then should you consider how to
couple energy into that antenna.
Dave, W1MCE
Rud Merriam wrote:
>> So we know that SWR isn't enough in determining a 'good' antenna. What other
> information is needed? Specifically, how do determine from a model if an
> antenna is efficient, i.e. putting the most signal into the air? Neglect the
> radiation angles and directions. While they are important they are readily
> obvious from the model, it seems.
>> I've been reading Cebik's web pages. There is an article -
> http://www.cebik.com/aledz.html - about an 'one antenna' for upper HF that
> is 44' long. At one point he comments that the impedance across all the
> bands is 'manageable'. I modeled the SWR across the bands and it seems
> non-manageable to me. Any comments on what to consider manageable? One
> criteria I'm looking at is the spec on my automatic antenna tuner which says
> it will handle an SWR up to 10:1 or impedances to 800 ohms.
>> Start at the basics with me, please. I have the NEC4WIN evaluation version
> to see if antenna work is something I want to explore. Plus I'm trying to
> find a good HF antenna for my location. It has to be stealthy but not
> terribly so. I do have areas where I could hid a vertical, say, or a
> horizontal wire. Also could probably put a delta, say, on the side of the
> house.
>> Rud Merriam
> K5RUD
>> - - -
>> Your moderator for this list is:
> Larry Wilson KE1HZ [email protected]
> _______________________________________________
> Antennas mailing list
> [email protected]
> http://mailman.qth.net/mailman/listinfo/antennas