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Author Topic: swr  (Read 206 times)

Posts: 32

« on: July 16, 2005, 03:59:33 PM »

I just ordered a new comet GP3 and 50ft of 9913 coax from HRO when I install this antenna what should I expect the swr to be?

Posts: 559

« Reply #1 on: July 16, 2005, 06:34:19 PM »

Most antennas that you buy are designed to be used with 50 ohm coax.  As a result, they are designed/made to also be around 50 ohms.

PS: you might find a lot of interesting info at W4RNL's web site.

Posts: 299

« Reply #2 on: July 17, 2005, 08:16:46 AM »

To answer your question, you should be able to get a decent SWR (less than 2.0:1) with that antenna setup if it is installed optimally.  There are many factors that effect SWR and it is not always a good indication of how well the antenna will radiate. Such as, a dummy load will have a perfect 1.0:1 SWR.  It just does not radiate very well.

First off, SWR (standing wave RATIO) is not measured in ohms.  It is the ratio of directed power to reflected power, so technically it has no "units" since they cancel mathematically.  Reflected power creates standing waves in the antenna system (antenna and feed line), hence the term standing wave ratio.  A low value (1:1) is ideal and anything greater is getting further away from ideal.

Many things influence SWR.  Antenna design is a major factor.  If the antenna design is such that it presents a purely 50 ohm resistive load (adds no capacitance or inductance component to the impedance; i.e. "resonant") to the transmitter designed for a 50 ohm load and there is no reflected power, you will get an SWR at the transmitter that is 1.0:1 and is 100% efficient in power loading (not to be confused with antenna efficiency).  Usually if this occurs, it will occur in a very narrow frequency range.  Antenna manufacturers want to make the antenna usable over a wide frequency range, so to will have an antenna that is near an ideal SWR (1.0:1) in the middle of the band for which it is designed.  Most antenna manufacturers show the 2.0:1 bandwidth, which is the frequency range of the antenna where the SWR is between 1.0:1 and 2.0:1  Be forewarned, some antenna manufacturers lie about their antenna performance or have poor construction QC that manifests itself as antenna performance values that do not match what the manufacturer claims (imagine that! HI HI)

Also included in antenna design is the difference between balanced antennas (such as dipoles, loops and yagis) and unbalanced antennas, such as verticals.  A vertical needs a sufficient and efficient radial system (counterpoise or ground planes perform the same function).  You will not get a low SWR on a vertical that requires radials with a poor radial system.

If the antenna is resonant (the impedance is purely resistive, 50 ohms, low SWR) on a particular frequency you are using, and you are using 50 ohm coax, then the coax will not effect the natural SWR of the antenna system.  This condition is referred to as "ideal" and is rarely obtainable in practical operation.   Usually the coax will affect the impedance, which has some effect on SWR.  Hopefully not too much.

The other thing that affects SWR is the location of the antenna and obstructions in the near field.  On HF it is not as much a consideration as it is with VHF and higher frequencies.  To be sure this is not affecting the SWR, make sure you have at least a 1-wavelength separation from the antenna and any metal or RF conducting materials.

If you have a non-resonant antenna system, you can try to trim the coax so that you change the voltage/current point that is being presented to the transmitter, which will slightly modify the SWR at a particular frequency.  I had to do just that on a turnstile-reflector antenna I am using for satellite communication on 2 meters since the antenna is very narrow banded with respect to SWR on 2 meters.  After trimming the coax, it has a great SWR (1.1:1) at 145.900 MHz and poor SWR at the band edges (2.0:1).  At the band edges with this antenna, the power on my transceiver cuts back to about 50% to protect the solid-state finals.


Posts: 2193

« Reply #3 on: July 17, 2005, 10:53:33 AM »

I just have to hop in and and say this, but if you're trimming your SWR by changing the length of your feed coax, you've got some serious common mode current issues. Grab any Smith chart and what you'll find is that SWR is independant of feedline length (excepting loss). If you find that your SWR is sensitive to cable length, you've got RF on your coax shield or some other issue.

What changes with feedline length is impedance, but the SWR is constant.

Also, resonance does not mean 50 ohms. It only means no reactance. The resistance can be almost anything. For instance, a dipole at resonance is 73 ohms.
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