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'[OT]: (Stupid) Antenna question..'
2002\12\29@080433 by Milan Pavlica YU7XW

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...Hi!
I need to made a quick dipole for 21MHz
I have only:
RG213 from radio-to-antenna
It is important not to use balun or any twin leads etc etc etc
Just simple wire conected to RG213
Any ideas?
What length of wire need to be?
THANKS!

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2002\12\29@082406 by Thomas C. Sefranek

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Dipole length in FEET = 468/Frequency.

SO:  468/21 = 22.282'  or   (3.57 Meters of wire on each end)

But you are a Ham radio operator, you know the 15 meter band lengths!

 *
 |  __O    Thomas C. Sefranek   .....WA1RHPKILLspamspam@spam@ARRL.NET
 |_-\<,_   Amateur Radio Operator: WA1RHP
 (*)/ (*)  Bicycle mobile on 145.41, 448.625 MHz

hamradio.cmcorp.com/inventory/Inventory.html
http://www.harvardrepeater.org

> {Original Message removed}

2002\12\29@090049 by Rick C.

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Wasn't this question part of your exam to get your license? The answer
should also be in your study guide.
Rick

Milan Pavlica YU7XW wrote:

{Quote hidden}

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2002\12\29@092445 by Thomas C. Sefranek

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Wow!  Ouch!
Rick, perhaps you need to LIVE in another country before you apply your USA
prejudice.

 *
 |  __O    Thomas C. Sefranek   WA1RHPspamKILLspamARRL.NET
 |_-\<,_   Amateur Radio Operator: WA1RHP
 (*)/ (*)  Bicycle mobile on 145.41, 448.625 MHz

hamradio.cmcorp.com/inventory/Inventory.html
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> {Original Message removed}

2002\12\29@095827 by Milan Pavlica YU7XW

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I know all theory, i calculated, and when i connected to my FT101Z, i had
SWR 1:3 and 1:4 !!!!
Where i got wrong?
I have dipole l/2 for 80m and works 100% - 3.737 SWR is 1:1.2 :)))))
Now, on 21MHz i have a problem...
Where i am getting wrong....

"Rick C." wrote:

{Quote hidden}

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2002\12\29@124523 by John Ferrell

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The math is all theoretical, a great place to start. Antennas, especially
transmitting antennas can be humbling.

My thoughts:
Is the antenna location to close to the ground?
Is the coax shorted or open?
Could you be coupling to something in the near field of the antenna that
affects its resonance?

Sometimes drooping the ends of a dipole will help bring the match closer to
unity.
Baluns are more popular than effective. My Cushcraft A3S Multiband beam is
fed directly but they specify that you use six turns of the coax on an 8
inch diameter as an rf choke at the antenna end. This discourages rf on the
outside of the coax.

How long is the coax run? It can make a difference.

John Ferrell
6241 Phillippi Rd
Julian NC 27283
Phone: (336)685-9606
Dixie Competition Products
NSRCA 479 AMA 4190  W8CCW
"My Competition is Not My Enemy"



{Original Message removed}

2002\12\29@133201 by Milan Pavlica YU7XW

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John Ferrell wrote:

> The math is all theoretical, a great place to start. Antennas, especially
> transmitting antennas can be humbling.

Really? I didnt see that ;-)

> My thoughts:
> Is the antenna location to close to the ground?

Nope. This is the story.
It was 1/2wave 3.5MHz diple which resonated very well.
Now i wanted to build 21MHz dipole, cuted the wire - and it's doesnt work.
Antenna is located under roof of my house, and coaxial cable length is about
10m - exactly above my room where radio is located is feed of antenna.

> Is the coax shorted or open?

open

> Could you be coupling to something in the near field of the antenna that
> affects its resonance?

i dont think so.
I just checked radio on dummy load and radio is OK

> Sometimes drooping the ends of a dipole will help bring the match closer to
> unity.
> Baluns are more popular than effective. My Cushcraft A3S Multiband beam is
> fed directly but they specify that you use six turns of the coax on an 8
> inch diameter as an rf choke at the antenna end. This discourages rf on the
> outside of the coax.
>
> How long is the coax run? It can make a difference.

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2002\12\29@135240 by Dale Botkin

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Dipole length will be affected by height above ground, nearby objects,
etc.  Also if you're feeding directly with coax you'll have some mismatch
to begin with.

That antenna as it is will be resonant at *some* frequency, just obviously
not the one you want.  Check SWR at both ends of the 15M band.  That will
tell you if the antenna is too long or too short - if SWR is lower at the
low end of the band, that means the resonant frequency is too low.  The
antenna is too long (lucky you) and you need to trim the ends a little at
a time until it's resonant in the 15M band.

If the SWR is lower at the high end of the band, the resonant frequency is
too high, and the antenna is too short.  Solder some wire (a foot or two)
on each end and start trimming.  Of course if you have access to an
antenna analyzer like an MFJ-259 or something, that will make your job
LOTS easier.

Just one suggestion - if you make that a 40M dipole, you can use it for
40M and 15M.  Since you have an 80 meter dipole, I assume you've got space
for a 40M as well...

73,
Dale - N0XAS
--
It's a thankless job, but I've got a lot of Karma to burn off.


On Sun, 29 Dec 2002, Milan Pavlica YU7XW wrote:

> I know all theory, i calculated, and when i connected to my FT101Z, i had
> SWR 1:3 and 1:4 !!!!
> Where i got wrong?
> I have dipole l/2 for 80m and works 100% - 3.737 SWR is 1:1.2 :)))))
> Now, on 21MHz i have a problem...
> Where i am getting wrong....

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2002\12\29@135652 by Thomas C. Sefranek

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O.K. Back to the basics...

SWR 1:4 (Actually 4:1)
I assume you swept the frequency, and this is the lowest SWR in the band.

That means you are seeing either 50/4 ohms, or 50 * 4 ohms.

I'd be real suspicious of Feed line radiation.
(The outside of the coax is part of your radiator.)
Do you have a choke flange?  (Current balun)?

If not, try wrapping a few turns of the feed line in a coil at the feed
point.
You can also wrap a few turns before the rig!
Then re-measure the SWR.

 *
 |  __O    Thomas C. Sefranek   WA1RHPspamspam_OUTARRL.NET
 |_-\<,_   Amateur Radio Operator: WA1RHP
 (*)/ (*)  Bicycle mobile on 145.41, 448.625 MHz

hamradio.cmcorp.com/inventory/Inventory.html
http://www.harvardrepeater.org

> {Original Message removed}

2002\12\29@141606 by Peter L. Peres

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On Sun, 29 Dec 2002, Milan Pavlica YU7XW wrote:

*>I know all theory, i calculated, and when i connected to my FT101Z, i had
*>SWR 1:3 and 1:4 !!!!
*>Where i got wrong?

Imho the best way is to use a vertical lambda/4 using thin wire (the thin
wire will up the impedance from 20ish to 50ish - I hope). You HAVE to
provide ground at the end of the coax where the stick starts. Solder it to
something large and metallic imho. or provide 'counterweights' (lambda/4
or longer wires, several, stretched horizontally or downwards. To prevent
the coax from radiating (probably the reason for your bad mismatch) you
can put several (5-10) ferrite toroids on the coax near the end with the
antenna. To make the antenna resonate wider you can connect it directly at
the base with a small T-pad. A 2dB T-pad will go a long way here. Remember
the T-pad will heat up when you transmit, use appropriate size resistors.
This is a hack not a proper match but you asked for a hack.

hope this helps,

Peter

*>I have dipole l/2 for 80m and works 100% - 3.737 SWR is 1:1.2 :)))))
*>Now, on 21MHz i have a problem...
*>Where i am getting wrong....
*>
*>"Rick C." wrote:
*>
*>> Wasn't this question part of your exam to get your license? The answer
*>> should also be in your study guide.
*>> Rick
*>>
*>> Milan Pavlica YU7XW wrote:
*>>
*>> > ...Hi!
*>> > I need to made a quick dipole for 21MHz
*>> > I have only:
*>> > RG213 from radio-to-antenna
*>> > It is important not to use balun or any twin leads etc etc etc
*>> > Just simple wire conected to RG213
*>> > Any ideas?
*>> > What length of wire need to be?
*>> > THANKS!
*>> >
*>>
*>> --
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*>> (like ads or off topics) for you. See http://www.piclist.com/#topics
*>
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*>
*>

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2002\12\29@152117 by John Ferrell

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Here is a couple more thoughts:
Reduce power for testing to about 5 to 10 watts. This is a safety measure in
case you are dumping power into something in the structure. House wiring,
plumbing, downspouts, etc, can easily be accidently resonant on 15M.

Your 10m of coax with a velocity factor of about 0.66 might be an electrical
half wave to this system. That would show your transmitter an infinite
impedance. If it is not too difficult, ADD about 5 meters of coax at the
trasmitter end to assure that it is not performing as a half wave line. Be
certain to not coil it so it becomes a choke.

I assume the antenna feed point is difficult to get to. If it is not, take
your dummy load to the antenna end of the coax and check the SWR. If that is
good consider a matching stub across the antenna terminals. It is messy to
set up because it takes a helper. I usually use twin lead tv antenna wire
for matching and an open line is easier than having to short the stub again
everytime you cut it. Test at low power for safety!


Dipoles are theoretically 70 ohms
Transmitters are usually 50 ohms
Most coax in amateur work is 50 ohms

AND all of these numbers only work when they are matched!


John Ferrell
6241 Phillippi Rd
Julian NC 27283
Phone: (336)685-9606
Dixie Competition Products
NSRCA 479 AMA 4190  W8CCW
"My Competition is Not My Enemy"



{Original Message removed}

2002\12\29@164219 by John Ferrell

face picon face
>Your 10m of coax with a velocity factor of about 0.66 might be an
electrical
> half wave to this system.
Oooops! That would be more like a full wave and less likely the problem. I
would still recomend adding a little coax to see the difference.

BTW, 1.4 : 1 is a pretty good match. I seldom worry with anything better
than 2:1.

John Ferrell
6241 Phillippi Rd
Julian NC 27283
Phone: (336)685-9606
Dixie Competition Products
NSRCA 479 AMA 4190  W8CCW
"My Competition is Not My Enemy"



{Original Message removed}

2002\12\29@181243 by Milan Pavlica YU7XW

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Dale Botkin wrote:

> That antenna as it is will be resonant at *some* frequency, just obviously
> not the one you want.  Check SWR at both ends of the 15M band.  That will
> tell you if the antenna is too long or too short - if SWR is lower at the
> low end of the band, that means the resonant frequency is too low.  The
> antenna is too long (lucky you) and you need to trim the ends a little at
> a time until it's resonant in the 15M band.

from 20.950~21.550 SWR is 4:1
Just checked

> If the SWR is lower at the high end of the band, the resonant frequency is
> too high, and the antenna is too short.  Solder some wire (a foot or two)
> on each end and start trimming.  Of course if you have access to an
> antenna analyzer like an MFJ-259 or something, that will make your job
> LOTS easier.
>

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2002\12\29@181447 by Thomas C. Sefranek

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> -----Original Message-----
> From: pic microcontroller discussion list
> [RemoveMEPICLISTspamTakeThisOuTMITVMA.MIT.EDU]On Behalf Of John Ferrell
> Sent: Sunday, December 29, 2002 3:20 PM
> To: PICLISTEraseMEspam.....MITVMA.MIT.EDU
> Subject: Re: [OT]: (Stupid) Antenna question..
>
> Your 10m of coax with a velocity factor of about 0.66 might be an
electrical
> half wave to this system.

> That would show your transmitter an infinite impedance.

Unless you believe the other end is a SHORT and the coax is lossless this is
WRONG.

> If it is not too difficult, ADD about 5 meters of coax at the
> trasmitter end to assure that it is not performing as a half wave line. Be
> certain to not coil it so it becomes a choke.

Why would you NOT want a common mode choke?
{Quote hidden}

 |  __O    Thomas C. Sefranek   EraseMEWA1RHPspamARRL.NET
 |_-\<,_   Amateur Radio Operator: WA1RHP
 (*)/ (*)  Bicycle mobile on 145.41, 448.625 MHz

hamradio.cmcorp.com/inventory/Inventory.html
http://www.harvardrepeater.org

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2002\12\29@204554 by Thomas C. Sefranek

face picon face
Wait!

The SWR doesn't vary over that range???  21-21.5 MHz!
Something is VERY wrong.

 *
 |  __O    Thomas C. Sefranek   RemoveMEWA1RHPEraseMEspamEraseMEARRL.NET
 |_-\<,_   Amateur Radio Operator: WA1RHP
 (*)/ (*)  Bicycle mobile on 145.41, 448.625 MHz

hamradio.cmcorp.com/inventory/Inventory.html
http://www.harvardrepeater.org

> {Original Message removed}

2002\12\29@213646 by John Ferrell

face picon face
> > That would show your transmitter an infinite impedance.
>
> Unless you believe the other end is a SHORT and the coax is lossless this
is
> WRONG.
>
No argument, I think an open or a short at the feed point most probable
failure. Even old coax has pretty low losses on 15M.

> Why would you NOT want a common mode choke?
One change at a time please, I am old and slow...

I have many years of experience with antennas. Unfortunately, a lot of it
was bad experience.

John Ferrell
6241 Phillippi Rd
Julian NC 27283
Phone: (336)685-9606
Dixie Competition Products
NSRCA 479 AMA 4190  W8CCW
"My Competition is Not My Enemy"



----- Original Message -----
From: "Thomas C. Sefranek" <RemoveMEtcsspam_OUTspamKILLspamCMCORP.COM>
To: <RemoveMEPICLISTTakeThisOuTspamspamMITVMA.MIT.EDU>
Sent: Sunday, December 29, 2002 6:12 PM
Subject: Re: [OT]: (Stupid) Antenna question..


> > {Original Message removed}

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