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Author Topic: Another mag mount CB to 10m question  (Read 4260 times)
KI4SDY
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Posts: 1452




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« Reply #15 on: January 26, 2012, 06:35:26 AM »

Success! You didn't even have to "trim" the whip, just like I said. Now you have an idea how this antenna would work on 10 meters when it is used as designed. You may even have a mobile antenna that will work well on 10, 11 and 12 meters, without spending any more money. Drive to nearest hill and work some DX! Grin

In addition, you have some good suggestions on how to build a separate inexpensive excellent performing base antenna, that will work in your treeless yard.  Wink

I look forward to talking to you on 10 meters.  73s!Grin  
« Last Edit: January 26, 2012, 09:21:48 AM by KI4SDY » Logged
W5LZ
Member

Posts: 477




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« Reply #16 on: January 29, 2012, 09:02:42 PM »

Can you make an 11 meter antenna work on 10 meters?  Sure, it's done quite often.  Since the two bands are so close, it's mostly a matter of 'trimming' the whip for resonance (10 meter antenna is shorter than an 11 meter antenna).  Will you end up with a 'perfect' SWR?  I doubt it.  At least not without some effort.  A 1/4 wave antenna isn't going to be 50 ohms anyway.  It should be reasonable though, something around 1.5:1.

There's always two steps with tuning an antenna, making it resonant and then matching impedances.  To do either of those 'steps' you need to use a measuring device that will tell you about reactance.  Resonance is defined as the absence of reactance.  An SWR meter wouldn't know a reactance if it was bitten by one.  So using an SWR meter to 'tune' an antenna isn't really very practical.  Does that mean that all them thousands of people tuning an antenna with an SWR are wrong?  Smiley  Yep.  Think about it.
 - 'Doc
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WB6BYU
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Posts: 13143




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« Reply #17 on: January 29, 2012, 10:04:29 PM »

Quote from: W5LZ

So using an SWR meter to 'tune' an antenna isn't really very practical.  Does that mean that all them thousands of people tuning an antenna with an SWR are wrong?  Smiley  Yep.  Think about it.



I have thought about it, and you are wrong.  The SWR meter is a perfectly sufficient tool to
tune such an antenna.

What is the object of tuning an antenna?  To present a load impedance to the transmitter that
is close to the load it was designed for.

What does an SWR meter measure?  How close the impedance is to 50 ohms (or whatever the
impedance the SWR meter is designed for.)  When the SWR is 1 : 1 on 50 ohm coax, the load
impedance must be exactly 50 ohms resistance with zero reactance.  Any other impedance
will give you a different SWR.

So here you have a device that tells you how close you are to the best match to the transmitter.
You use it to adjust the antenna to get the best match.  What are you proposing as an alternative -
something that doesn't give the best match?  Why would anyone in their right mind want such
a thing?

In this case the antenna is a base-loaded whip with the feedline tapped up the coil for impedance
matching.  It is designed so that, with the proper length of whip to resonate on 27 MHz it gives a
good match to 50 ohms.  Now we are shortening the antenna to make it resonant at 28.5 MHz.
To do so we adjust the whip for the lowest SWR - that tells us that we have the best possible
match for that particular antenna.  Because the frequencies change is small, the impedance
matching is still very close, and we expect that the SWR will be low.  (Clearly in this case the
ground plane it is mounted on had a very large effect on the impedance, but it could still be
tuned for minimum SWR by adjusting the whip.)  So if you adjust the whip for minimum SWR
the transmitter sees as close to its design impedance as you can get.

If the SWR meter gives you the best possible match, what could do any better?


[And before you go off on a tangent about resonant antennas, be sure to read , reflect, and digest
some of the articles available on the internet about how important - or not - resonance actually is
to antenna performance, and the fact that a pure resistance at some point on the feedline doesn't
guarantee that the antenna is, in fact, resonant.

Such as these:

http://www.vk1od.net/antenna/iobr/
http://www.cebik.com/content/a10/trans/x.html
http://www.vk1od.net/antenna/arrm/index.htm

This is to help save you from sticking your foot any further in your mouth than you have already.]
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KW6LA
Member

Posts: 91




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« Reply #18 on: January 31, 2012, 12:27:14 AM »

 Read this again ! !

There's always two steps with tuning an antenna, making it resonant and then matching impedances.  To do either of those 'steps' you need to use a measuring device that will tell you about reactance.  Resonance is defined as the absence of reactance.  An SWR meter wouldn't know a reactance if it was bitten by one.  So using an SWR meter to 'tune' an antenna isn't really very practical.  Does that mean that all them thousands of people tuning an antenna with an SWR are wrong?    Yep.  Think about it.  Also people just dont think about the gound plane being the other part of the antenna.

 Perfect Doc !
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W5LZ
Member

Posts: 477




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« Reply #19 on: January 31, 2012, 06:42:03 AM »

WB6BYU,
The 'taste' of 'foot' isn't all that bad, you know?  'Course, it depends on the 'foot', and why you are 'tasting' it!  I'm not 'tasting' any 'foot' at the moment.  Smiley
 - 'Doc
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WB6BYU
Member

Posts: 13143




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« Reply #20 on: January 31, 2012, 07:04:51 AM »

But if the SWR meter tells you how close you are to 50 + j0 ohms, which is the load that transmitter
wants to see, and the antenna is adjusted for 1 : 1 SWR, then you have achieved a perfect match
by simply adjusting the antenna for lowest SWR.

If you only have one adjustment, you tune the antenna for lowest SWR at the desired operating
frequency.  If you have two adjustments (that might be R and X, or phase and magnitude, or two
interrelated adjustments that affect both R and X as might be the case with an antenna tuner) then
you adjust them alternately for lowest SWR. 

In fact it is rarely the case that the R and X components of impedance are adjusted independently, as
most adjustments affect both.  Trimming a dipole, for example, shifts the reactance more than the
resistance, but both vary with length.  In practice one simply adjusts the length of the antenna for
minimum SWR at the frequency of interest.


You have not yet shown how you would apply your method to any practical antenna, or how it would
achieve a result any better than would be obtained using just an SWR meter.  If it doesn't obtain a
result better than 1 : 1, as an SWR meter can, then you can't say that using one is "wrong".


So start with a simple dipole and explain how you would tune it other than adjusting the length for
minimum SWR, and why that gives you a better result than what is achieved with an SWR meter.

Then you can explain how you would adjust the CB antenna in question that has a single adjustment
for whip length and get a better result [to bring us back to the original topic of the thread.]
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