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by0ung(at)brigham.net
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 Posted: Mon Jan 29, 2007 9:06 am Post subject: antennas |
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This is from my last email sent to Larry Cottrell, I hope someone can use
the information.
Boyd
I have a cut sheet for antenna's and it says 23.5. The part I am not sure of
is whether the center conductor at 23.5 and the braid pointed the other
direction at 23.5 is the correct way to go. As it is I mounted my old
antenna up on the top of the wind screen close to the wings, and it really
doesn't do that badly. I am going to make one of your antennas and use it on
the portable that I leave with Karen. First however I will try it on the
plane and if it performs better than the outside antenna, I will make
another for the wife.
Thanks
Larry
...............
Larry
23.5 would be correct and that is what I have seen in some aircraft
magazines…. But it depends on the type of wire you are using.
It seems that every piece of coax has its very own “velocity factor”
which is stated as .7 .75 .78 .8 etc…. the velocity factor is a
percent of the speed of light. If the velocity factor is .9 my antenna
would have to be longer than an antenna of my design with a coax of velocity
factor of .7 that Is why you have to measure the swr or standing wave
ratio in order to properly tune it. In receive it wont make much
difference,,,, but in transmit the worse case is it will over temp the final
transistors and burn them out. To picture what a swr is, tie a rope to a
door knob.( visualize the rope as if it were the antenna) Stand across the
room and pull the rope almost tight and then with a flick of the wrist in an
up and down motion watch the pulse in the rope travel up the rope hit the
far end then come back to your hand. The amount of energy your hand has to
absorb would be absorbed by the final transistors. Now I suppose if you
could get the rope the exact length, when you gave your end a flip, the
pulse would travel up the rope and just as it hit the other end all of the
energy from the flip was used up and nothing come back then the antenna
would be in resonance to the frequency. Now in a rope a longer rope
would use up all the energy, but in a wire antenna longer or shorter will
cause the back lash. A longer coax between the radio and antenna would
just use up energy and be of no use.
For a handheld radio one of the easiest ways to extend the range with a
rubber duck antenna or with an telescopic antenna, { if you use a
telescopic antenna you need to figure the proper distance to telescope it
to} you could go to Ham Radio Outlet… hro.com and find what is called a
tiger tail…. Basically it is an eyelet with a piece of wire attached to it
that goes under the bnc connector of the antenna, it works the same as the
braid hanging out. Very nice and portable, but if you want an antenna
you can hang in the living room, my diple would work well.
Another antenna you could use at the house would be a “J Pole” antenna
Hope this diagram works.
................o..............................
\
........o../
The left side is the top and the right the bottom and can be mounted to a
steel pole or direct buried into the ground. The coax hooks into the
antenna where indicated by the “o”. it is best if made of ½ or ¾ inch
copper pipe. The length of the 2 upper pipes and the exact positioning
of the “o” is how you tune the antenna. The bottom length can be any
distance. This is a great antenna for outdoors because it is short
circuited to ground in case of lighting strikes.
Boyd
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a58r(at)verizon.net
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| Posted: Mon Jan 29, 2007 3:38 pm Post subject: antennas |
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Several years ago there was a great hue and cry about trimming/
adjusting VHF antennas and their associated transmission lines/coax
for exact SWRs. This so-called "problem" is greatly overrated at the
transmitter power levels found in our powerful 1-5 watters. And the
same obtains with the associated receiver. SWRs of 2:1, even 3:1
won't "blow" the final amp in the xmtr; nor will there be much
perceived loss of rcvr sensitivity. Antenna placement and altitude
are much larger factors.
Of course it's been almost 58 years since BSEE...could be rong.
regards,
Bob N. FireFly 070 Old Kolb
http://www.angelfire.com/rpg/ronoy/
do not archive
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APilot(at)webtv.net
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| Posted: Mon Jan 29, 2007 5:13 pm Post subject: antennas |
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I gave up trying to transmit. I could hear okay with my exterior
antenna, but most could not hear me very good.. Recently, I purchase an
inside antenna from a place in Canada. I think that it is called a
magic air. Anyway, it is suppose to be pre-tuned and is mounted inside
the fiberglass nose cone. If anyone has tried one let me know how it
worked. I will be installing mine and testing it next week. Fingers
are crossed. Vic in Sacramento
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herbgh
Joined: 10 Jan 2006
Posts: 145
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| Posted: Mon Jan 29, 2007 5:30 pm Post subject: antennas |
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Vic
As I recall.. poor range even with a good antenna, can sometimes
indicate a blown final...ie: output stage transistor.. What one is
hearing at short range is some small amount of radiation from the driver
transistor.. FWIW Herb
do not archive.
On Mon, 29 Jan 2007 17:12:55 -0800 APilot(at)webtv.net writes:
| Quote: |
I gave up trying to transmit. I could hear okay with my exterior
antenna, but most could not hear me very good.. Recently, I
purchase an
inside antenna from a place in Canada. I think that it is called a
magic air. Anyway, it is suppose to be pre-tuned and is mounted
inside
the fiberglass nose cone. If anyone has tried one let me know how
it
worked. I will be installing mine and testing it next week.
Fingers
are crossed. Vic in Sacramento
|
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by0ung(at)brigham.net
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| Posted: Tue Jan 30, 2007 9:39 am Post subject: antennas |
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Several years ago there was a great hue and cry about trimming/
adjusting VHF antennas and their associated transmission lines/coax
for exact SWRs. This so-called "problem" is greatly overrated at the
transmitter power levels found in our powerful 1-5 watters. And the
same obtains with the associated receiver. SWRs of 2:1, even 3:1
won't "blow" the final amp in the xmtr; nor will there be much
perceived loss of rcvr sensitivity. Antenna placement and altitude
are much larger factors.
Bob N.
You are exactly correct with regard to 2 to 1 and a properly tuned
antenna at 1 to 1 in the center of the band will probably have a 2 to 1 at
each end... but 3 to 1 is a stretch for me... and for those who don't
check it.... who knows what it really is.
Boyd
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