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Author Topic: trouble tuning a home brew bug catcher  (Read 3440 times)
FROG
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Posts: 55




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« on: July 04, 2012, 10:22:51 AM »

 I made a home brew bug catcher from an article on the arrl web site....did everything as advised but can not get it to tune. I have a 259b analyzer but when I touch the antenna  at the taps points or just get close to it the readings go all over.Either the r is to high or the srw is to high or the x is way out.....when I get the swr down then the r factor is way  to high. Driving me crazy..Moved the car into the clear and no change.  I move the match tap and then the load tap and just cant seem to get it right. The article said 20-17-15 all tuned very easy.. Ive lengthened the whip and shorten it....well not in my case...Any Ideas....I'm about to give it up and buy a dam antenna....I have also checked all the grounding cables ...all seems well..1 inch straps on the mount..grounded good...
Thanks
Frog
« Last Edit: July 04, 2012, 10:30:50 AM by FROG » Logged
WB6BYU
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Posts: 13586




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« Reply #1 on: July 04, 2012, 12:24:28 PM »

Are you standing in proximity to the antenna while tuning it?  Your body will detune the
antenna - after you adjust it you have to move out of the field to measure the effect.
I've done this laying on the ground under the back bumper if the adjustment is near
the bottom of the antenna.

Do you have a length of coax between the antenna and the SWR analyzer?  If so
you can't trust the R and X numbers (unless they are corrected for the exact length
of the coax.)  In that case just tune for best SWR at the desired frequency.

It sounds as though you have a loading coil (probably in the middle of the whip)
and a matching coil across the feedpoint.  I'd suggest disconnecting the matching
coil for initial tuning of the loading coil - just get it to where the SWR dip is in the
desired band.  That is a good starting point at any rate.  Then attach the matching
coil and adjust it for minimum SWR at the dip, though the dip will change frequency
somewhat.  Once you get that, you can readjust the loading coil to move the
resonant frequency where you want it.
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W5FYI
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« Reply #2 on: July 04, 2012, 06:35:27 PM »

I agree with Dale; trying to tune a mobile antenna while standing next to it is somewhat frustrating. The ideal way to do it is from inside the vehicle, while it's tooling down the highway, primarily because that's where it will be used most!

Typical mobile antennas should have low r (radiation resistance) and low feed point resistance. The loading coil is there to cancel the capacitive reactance of short mobile antennas, so reactance should be as close to zero as possible. When you get zero X, then you can start matching the feed line to the antenna's r.

Try to find an open area away from other vehicles and overhead utility lines to run your tests. You also need to be inside the vehicle, with the doors closed, when you measure the antenna's parameters. I would start with the whip almost fully extended and the matching coil bypassed, then start adjusting the loading coil tap until you get close to zero reactance.  From that point on you can fine tune the matching coil and loading coil to get close to 1:1 SWR at the rig end of the feeder.

I would also recommend you use a mobile antenna design program to find good starting points for loading and matching inductances. The ARRL Antenna Book is a good starting point as well.

I would also suggest that you not "short" out turns with your tap wire--the shorted turns will change the coil's inductance and will waste power. Keep one end of the coil open-circuited if at all possible. And, if you're using a capacity hat, mount it as far above the loading coil as possible.

If you can find it, Don Johnson's,W6AAQ's, book. 40+ Years of HF Mobileering is an excellent reference for building, testing and using mobile antennas. GL
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K5LXP
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« Reply #3 on: July 05, 2012, 06:10:12 AM »

The installation can have a significant impact to tuning.  It matters a lot where and how it's mounted and fed. What was the specific article from the ARRL site you used?  What are your installation specifics?


Mark K5LXP
Albuquerque, NM


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FROG
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Posts: 55




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« Reply #4 on: July 05, 2012, 07:23:50 AM »

Thanks for all the suggestions...I will give them a try .
Yes the antenna has a matching coil on the bottom and about 3 feet of shaft above and then a loading coil and then the whip.
It was from an article on the arrl webs site ..I just typed in home brew bug catcher and it came up. think it said something about a mobile $20 dollar antenna bugcatcher...something like that...anyway thanks again for the help..I will try again.
73
Frog
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WB6BYU
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« Reply #5 on: July 05, 2012, 08:21:31 AM »

A quick check on tuning is to tie a string near the top of the antenna.  If pulling it
away from the car lowers the SWR, you need less inductance.  If pulling it towards
the car lowers the SWR, you need more inductance.  If both cause the SWR to
increase, you tuned to the right resonant frequency (at least while the car is
parked.  The whip usually moves back a bit when the car is in motion.)
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W9PMZ
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« Reply #6 on: July 05, 2012, 08:47:27 AM »

if you have a matching coil at the bottom, disconnect it.

now tune the antenna and loading coil, tune for minimum X (=0, resonance), probably will have a low R depending on the frequency.

once you have tuned the loading coil then tune the matching coil at the base of the antenna.

73,

Carl - W9PMZ
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KF4ZGZ
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« Reply #7 on: July 05, 2012, 08:59:56 AM »

Sounds like it could be a grounding issue.
Double check all your grounds.


Matt
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K5LXP
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« Reply #8 on: July 05, 2012, 01:36:41 PM »

What are your installation specifics?

  - What vehicle
  - how mounted
  - how fed
  - what band(s)
  - what radio/how installed

It can be assumed if you built the antenna per the article that it will work predictably.  The fact that in your case it isn't can only be ascribed to two things - you didn't build it right or it's something about your installation or test method.  Given that many HF mobile installations are botched, these are the first variables to examine.


Mark K5LXP
Albuquerque, NM
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N3OX
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« Reply #9 on: July 05, 2012, 01:50:26 PM »

It is 100% absolutely normal for a Bugcatcher antenna to be nearly un-tuneable if you keep touching it or keep your body near it the whole time.  You need to move taps and back off.  This is completely normal.

I can wave my arms around six feet from my base loaded 160m antenna and play the MFJ-259B needle like a theremin.

There could be other problems, but even a perfectly functioning short antenna will be very sensitive to the presence of a big sack of water like a person... so you need to take that into account as you tune.

The procedure some have suggested where you disconnect the matching coil, adjust the loading coil for resonance, and put the matching coil back and adjust for best SWR is the best procedure for getting the thing dialed in.  But you definitely have to back off while you measure.
« Last Edit: July 05, 2012, 01:52:07 PM by N3OX » Logged

73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
KE3WD
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Posts: 5689




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« Reply #10 on: July 05, 2012, 02:52:51 PM »

Listen to the sound advice from N3OX there. 

Bugcatchers are notorious for wanting to capacitively couple to just abouy everything that might be even remotely conductive that is in their nearfield.  And that includes YOU.


73
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K9FV
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« Reply #11 on: July 05, 2012, 08:15:21 PM »

Well Dan - I learned something today.... so it's a good day.  theremin - I'd never heard/read that term before.  I had to look it up.  You choice of musical instruments was very apt..... waving arms around to change/play.

73 de Ken H>
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FROG
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Posts: 55




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« Reply #12 on: July 06, 2012, 07:54:04 AM »

Thanks for all the advise....I tried disconnecting the matching coil to tune the loading coil...but it doesn't tune at all unless the matching coil is connected...checked all connection and all are good...reread building info..all is good...grounds all good...cant get anything unless the matching coil is attached on a tap?Huh??
Frog
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WB6BYU
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« Reply #13 on: July 06, 2012, 08:47:58 AM »

I presume this is the article you used:

http://www.arrl.org/files/file/Technology/tis/info/pdf/0004033.pdf

In that case, connect the clip to the top of the matching coil.  That particular design
runs the hot lead to the coil tap rather than to the antenna as is done in most cases.
That means that disconnecting the tap disconnects the antenna from the coax.

A few checks you can do:

1) Use an ohmmeter to measure the resistance from the top whip of the antenna
to the car frame.  (You may need to extend the leads with some extra wire to do
this.)  when the clips are attached to the coils, you should read a low resistance.
Grounding is critical for HF mobile antennas, as KF4ZGZ pointed out.

2) Then measure the resistance to the shell and center conductor of your coax
at the radio end.  Both should show a low resistance.  When you disconnect the
clip from the matching coil the center conductor should show an open circuit.

3) Answer K5LXP's questions about how the antenna is mounted - that affects
the ground connections as well.  For example, a mag mount on a trunk doesn't
provide sufficient ground HF to work well at all, and you can expect to have
problems tuning it.

4) You have an SWR analyzer:  set the whip to full length, and the clips on both
coils to the top turns.  Tune across the upper HF band and find the frequency
(or frequencies) where the SWR dips, even if the minimum is 3 : 1 or so.  Then
move the tap on the loading coil down to the middle of the coil and repeat the
procedure.  Keep out of the field of the antenna while doing this - laying on
the ground allows you to use a very short jumper to the SWR analyzer.  (If
you can use a jumper less than about 1' long, then looking for the points
where X = 0 may provide useful information.)

Ideally you would check the antenna up to 80 MHz or so:  a resonance in the
range of 70 - 75 MHz would suggest a bad connection to the loading coil.
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FROG
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Posts: 55




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« Reply #14 on: July 06, 2012, 01:22:22 PM »

 OK I want to thank everyone for their input on this project...all the info came into play in the tuning process....I finally found a spot on the taps that led to the overall tuning of the rest....I guess you were all right...it just takes patients...and a lot of it..ha...anyway got it all tuned up ....and the swr is a lot better than I thought it would be...almost all the bands tuned below1.4 ................for 20-17-15  i had to have the whip out to 45".....and for 12-10 I had to retract the whip to 33".....
 I sure am glad it all worked out...now the fun part begins....to see how it actually puts out....
 Cant thank you all enough......
Frog  73s
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