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Author Topic: antenna won't tune  (Read 475 times)
KC8YFT
Member

Posts: 3




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« on: February 10, 2006, 09:09:59 AM »

here's the deal, i have an ic-706, a homebrew pi tuner, and a long wire in the backyard.  the first attempt worked "great", (i could get close to a 1:1 match almost anywhere except 160m)it was just a 4ft piece of coax, wire-nutted to the end of my 100ft long wire.  the antenna would tune all the way down to 80m, but i could pick up some rfi from the tv.  yes, from the tv.  so i decided to move the feedpoint from inside the qth, to outside via 50ft of rg-8.  this cured the tv/fri issue, but (and i knew it would) created a new issue.  now the antenna won't tune at all.  i suspected the ground.  so i attached a "mobile" style antenna mount to the ground rod, but to no avail.  then i added more wire to the length of the antenna, as a long wire is supposed to be over one wavelength long.  so i added another "bit" of wire.  now the length is roughly 175-200 ft.  still, i can't get the swr as low as it was before the feedpoint relocation.  still puzzled, i tried to make a bal-un.  i pulled this big nasty toroid from an old car amplifier.  it measures close to 2.5in od and a 1.5 in id.  i got 20 or so bifilar turns of 18 awg.  this did nothing, so i unwrapped one of the leads about 10 times.  now i can tune on 80m, at a 1.3:1, but i shouldn't need a bal-un and a tuner both...right?  what's the real problem here?  is my wire too long?  the way it's fed, it acts similiar to a 1/4 mobile, and as i add inductance (via tuner) the swr gets worse.
thanks in advance for reading all of this, i feel it's better to have too much info, rather than not enough.

73
jack
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WB6BYU
Member

Posts: 13143




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« Reply #1 on: February 10, 2006, 12:35:20 PM »

The problem is that you have the tuner at the rig end of
the coax rather than at the antenna.

Remember that any feedline will act as an impedance transformer
when the SWR on it is not 1 : 1.  (Unless it is exactly
a multiple of half a wavelength long.)  In your case,
50' will be close to 1/4 wavelength when you consider the
velocity factor of the coax cable.

Now, let's say your antenna was around 130' on 80m.  We
can expect an impedance of a few thousand ohms at the end
of the wire.  Let's choose 2500 ohms as an example because
it makes the calculations easy.  A tuner can match 2500
ohms with reasonable components.  But when the coax is
connected to the antenna, assuming a good ground
(which may not be the case, but would only make things
worse), the SWR on the coax is 2500/50 = 50 : 1.

This cable is about 1/4 wavelength long, so it will
transform the impedance to 2500 / ( 50 * 50 ) or 1 ohm.
If you check the formulas for a pi or L network to
match 1 ohm to 50 ohms, you will find that it requires
a LOT more capacitance than your tuner probably has.
And, although some of the "T" tuners can nominally match
impedances this low, the efficiency may be low.

So I suspect you will find that you can match the antenna
if you put your tuner at the junction between the coax
and the antenna wire.  This isn't always convenient to
adjust, of course.  You may find that changing the
length of the coax cable helps, at least on some bands.
(This is ignoring the issue of losses in the coax cable
at high SWR.)
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KC8YFT
Member

Posts: 3




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« Reply #2 on: February 10, 2006, 02:06:05 PM »

ok...that kinda makes sense.  putting the tuner outside (at the feedpoint)would indeed be optimal, but not practical, if possible.  as another option, or form of tuner, the bal-un, may help...right?  the key is, if i'm not mistaken, to match the antenna at the antenna feedpoint, not at the radio.  maybe i can move the feedpoint, use a shorter (or none at all) length of coax.  if i use, say, 5-10 ft of coax, would that be better by a littleor a lot?
thanks in advance.
jack
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VE3IOS
Member

Posts: 79




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« Reply #3 on: February 13, 2006, 08:14:57 AM »

If you added 50' of coax then take 50' of wire off your antenna.
Jeff
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KC8YFT
Member

Posts: 3




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« Reply #4 on: February 13, 2006, 06:55:49 PM »

ok...i moved the feed point closer to the "tuner", and am using only 6' of coax.  now i get a better match, however, now i have another problem.  i am picking up interference/noise, which i believe to be from the power lines that run up and down the street.  this is primarily evident on 160 and 80m, and there seems to be "lulls" in the offending hash.  it's not too noticeable in the higher frequencies (i think the reciever may be filtering them out)  i believe this is due to the fact that the first leg of my antenna runs parallel to the power lines (for 25' or so).  aside from re-routing the antenna, is there any other way to "filter" or "trap" the noise?
or is the simplest/most effective (and most obvious)way to move the antenna?
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WB6BYU
Member

Posts: 13143




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« Reply #5 on: February 14, 2006, 10:22:35 AM »

Are you sure this is from the power lines?  A quick check
is to put the rig on battery power (if possible) and
throw the main circuit breaker to your house.  Likely
the noise is coming from a computer, touch lamp, TV, alarm
system, or a switching power supply in some piece of equipment,
and generally it will go away when you turn off the power.
(There have been some cases where the interfering equipment
has a backup battery.)

So, before you try moving the antenna, I'd suggest making
sure that the noise really is coming from the power lines.
If so, you can call the power company and complain (which
may or may not get the problem solved in a reasonable time.)

You might want to wander over to the RFI forum and see
what folks have suggested for tracking down interference.
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