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Author Topic: SWR vs antenna gain  (Read 4517 times)

Posts: 22


« on: September 11, 2000, 07:57:22 PM »

Insulated backstay antenna on a sail boat.  I modeled it in EZNEC.  By changing the feed point I can vary the SWR at the frequencies I use from about 2 to infinity.  Changing the feed point also dramatically changes the radiation pattern.  I get a better take off angle with a middle feedpoint: 35 degrees with a gain of 6.69 dBi, but then the SWR for 20 meters is 10+.  On the other hand, with the feed point 15% of the way from the end the SWR is about 1.6 but the take off angle for the strongest part of the lobe is at about 45 degrees and the gain at that is only 2.74 dBi.

I guess the question is: if the tuner will tune the antenna at the higher feed point I would be better off with the good radiation pattern, right?

Am I loosing anything by using a tuner instead of varying the feed point to where the antenna is naturally resonant at the frequency I need to use.

Posts: 3585

« Reply #1 on: September 11, 2000, 10:07:53 PM »

 Well, there is a power loss if the SWR is above 1:1. Assuming all the reflected power is lost - you will lose 10% of your power if the SWR at the load, the antenna, is 2:1. However - that is less than 1 decibel. And 1 decibel is the smallest change in loudness that can be detected by the human ear. Therefore, as a practical matter, the amount of lost power is insignifigant, provided the SWR at the load is low enough, or if most of the reflected power is re-reflected at the rig and eventually radiated. So it would be more important to have a good radiation pattern with a reasonably low SWR than to have a poorer radiation pattern with a "perfect 1:1" SWR. However, a "poor" radiation pattern with one good low angle lobe might be just the ticket at times, while a nice round pattern might be perfect at other times. Which is best depends on what you want to do. It's also a good idea to remember that the indicated SWR at the rig/tuner will be a good bit less than the SWR at the load. How much less depends on how much loss there is in the co-ax and the length of co-ax.

Posts: 9

« Reply #2 on: September 11, 2000, 11:57:10 PM »

You are describing a net change in "gain" of about 4 dB  (6.69 - 2.74= 3.95 dB).  There are many ways at looking at this...

First, the smallest change in level that the human ear can ascertain is 3dB, not 1 dB.  So, you do get an additional 1 dB of usable gain in the low takeoff angle direction.  This represents about a 10% stronger signal but much less than an S unit.  However you are not cloud burning but radiating energy at a low takeoff angle, which is more important.

Second, and this would have to be verified; if you were to use an efficient, well designed antenna tuner, than only a portion of the reflected power would be lost as heat.  The balance would be radiated, but potentially "out of phase" with the original signal, nevertheless it has to go somewhere, which is that it gets radiated.  The difference in phase is small and should be of little consequence.  

From my perspective, which is somewhat less than scientific, DX communications would benefit more from the lower takeoff angle then from anything else.  Local communications would not require the antenna tuner, just moving the connection to the antenna.

Mitch Ravitz

Posts: 22


« Reply #3 on: September 12, 2000, 10:26:54 AM »

As you probably have become aware of using EZNEC, antenna performance (strictly speaking in terms of gain, directivity, front to back, etc.) has absolutely nothing to do with SWR.  SWR is merely an indication of the match (or mismatch) of the antenna system to the transmitter.

A higher SWR will result in increased power loss on the feedline (read: heat creation)-- and that loss is directly related to the quality of the feedline and the efficiency of the antenna tuner.  Notice how warm an antenna tuner becomes when translating a big mismatch to 50 ohms!

What has been done for years and years is to utilize the excellent characteristics of open-wire feedline to minimize the loss between the antenna tuner and the antenna -- resulting in designs like the G3RV doublet antenna, which inherently have high SWR to 50 ohms.  Open wire feed has very very low loss compared to even the best coaxial feeds, like 9913 -- the trade off being a mechanical one.  Open wire feed has to stay away from metallic objects and can be a bit of a problem when trying to route into the shack.

A 50 ohm match is convenient. That's the bottom line.

73 and good luck!

Rush -- W4QA


Posts: 22


« Reply #4 on: September 12, 2000, 10:28:37 AM »

Sorry.  G5RV, not G3RV.


Posts: 17406

« Reply #5 on: September 12, 2000, 01:57:45 PM »

There are lots of great antennas which don't happen to have a feedpoint
impedance of exactly 50 ohms.  A half-wave, end feed vertical is one
example (SWR 20+ relative to 50 ohms).  However, a good matching
network (or tuner) will enable an efficient power transfer.  That is why
you have a tuner - to match the antenna (whatever the impedance may
be) to 50 ohms for your transmitter.

No, reflected power isn't lost.  In fact, it may not even exist at all.  If you
put the tuner at the base of the antenna, instead of running coax between
them, and the tuner perfectly matches the antenna to the coax, then
there is no reflected power in the whole system, regardless of the
difference between the antenna input impedence and the impedance
of the coax.  If you are using feedline between the antenna and the
tuner, then it is more accurate to say that the reflected power goes back
and forth a couple times before it gets radiated, rather than that it is lost.
A good tuner should be 98% efficient or better.  Some tuners are not well
designed, and it is often possible to adjust even a good tuner for high
circulating currents, which will increase the loss.

Yes, open wire line is more efficient at high SWR, but I wouldn't suggest
it on a sailboat.

I suspect the problem you are encountering is that your backstay is over
a half wavelength long at the frequency you are modeling.  This would
account for the high angle radiation.  (A vertical one wavelength tall
would have a maximum lobe at about 45 degrees.)  If you are most
interested in 20 meters, for example, I suggest you add an insulator in
the middle of the stay about 30' up, and feed it with a tuner at the bottom.
Make sure you have a good ground system on the tuner.

Hope this helps.  Happy sailing.    - Dale  WB6BYU
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