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Author Topic: Antenna Help  (Read 1177 times)
KF6YB
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Posts: 6




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« on: June 19, 2002, 02:03:00 PM »

Hello,
I am fuzzy on this topic so here goes my question. I just finninshed a QRP Kit and now need to come up with a field portable antenna. One that I can take back packing and camping, It is for use on 40 Meters. I am thinking about an inverted vee with coax feeding a 1:1 balun.
Now, given that this antenna will be set in a variety of locations and geographic areas, the SWR will vary due to changes in height, proximity to trees and such.
Shall I use a match box to tune the mismatch?
Should I ignore the mismatch?
Is there a Tuner that I could build from a schematic to do this? (Money is in short supply)
I welcome any thoughts on this topic as I'm sure its not a new one to many of you.
73'
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WB2WIK
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« Reply #1 on: June 19, 2002, 02:58:21 PM »

If you're going to stick to working 40 meters, no environmental change should radically shift the antenna's resonant frequency.  

I've built a 40m 1/2-wave dipole with ~50' of permanently-attached RG58C/U to make a lightweight "rollup" antenna for portable/camping/etc. and no matter where or how I install it, at 10' above ground or 50' above ground; between trees, or simply sloping out of one; as an inverted vee, an upright vee, or a sloping vee; I've never had to use an antenna tuner with it.  Its resonant frequency changes, to be sure, but not more than a few hundred kHz, and to accommodate the changes I simply adjust the length of the dipole elements (wire legs) for minimum SWR at my frequency of interest, which is usually about 7040 kHz.

A simple, inexpensive and compact SWR bridge is all I use.  My dipole legs are 35' long each (to each tiny end insulator, where strings attach), which is a bit on the "long" side, just in case.  Normally, I have to adjust them down to 31-32' per leg to achieve resonance at 7240.  I do so by simply pulling the bare wire through the insulators a bit more, and wrapping the wire back on the element.  An entire "major adjustment" can be accomplished in about two minutes.

WB2WIK/6
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AD5IU
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Posts: 54




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« Reply #2 on: June 23, 2002, 06:08:29 AM »

Check this site out:

http://www.qsl.net/kq6xa/antenna/
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KB0KOQ
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Posts: 31




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« Reply #3 on: July 13, 2002, 08:15:14 AM »

WB2WIK

   I'm wondering about what you wrote. One question has always baffled me.  When we measure our dipole length, and run wire through the end insulator, turning the wire back upon itself and wrapping around the antenna for mechanical grip, are we altering the antenna length?   It is only the horizontal length of wire that matters?  Wrapping around at the end alters nothing or does it? (I know very little about antennas, and all the talk about BALUNS and what not really throws me).
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WB2WIK
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« Reply #4 on: July 15, 2002, 01:43:46 PM »

If you use bare wire (no insulation), then wrapping it back on itself sets the antenna length to be the point at which the wrapped back wire connects to the main horizontal (or whatever) radiating element.

If you use insulated wire, that's not the case.  The entire length of wire is still active and part of the antenna, even if you fold it back, although it's difficult to predict precisely what the antenna's electrical length will be.

None of this is particularly important, since so many variables impact an antenna's resonance.  Having an SWR bridge, or Antenna Analyzer, or both, goes a long way towards making antenna adjustments.  Physical dimensions alone don't do it, although they'll get you close.

WB2WIK/6

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KB0KOQ
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« Reply #5 on: July 16, 2002, 07:55:53 PM »

WB2WIK/6

   Okay.  I just read something about this in the Radio Shack "Antennas"  book and its called "end effect" and they told about the difference between physical antenna length and electrical length.  Thanks for helping.
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WB6BYU
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« Reply #6 on: September 04, 2002, 04:14:15 PM »

My experience is quite similar to Steve's...

I tune up my portable antennas supported about 12' off the ground,
with the ends sloping down to 6' or so (whatever is handy to tie off
to the bushes.)  Then I just put up the antenna and use it, and
don't worry about the SWR.  If the antenna is fairly low, it is well
matched, and if it is high in the air, then it gets out better, even if
the SWR is not perfect.  My dipoles have been hung as slopers,
inverted vees, vertical dipoles, propped up with a stick, and hung
between rock craigs with a 300' drop underneath.  The only time
the SWR was too high to operate on the 40m dipole was when it
was hung lengthwise along a barbed wire fence about 3' off the
ground (but I managed a contact with my HW-8 on 20m where the
SWR was lower.)

Oh, yes...  I don't bother with a balun, as it just adds more weight.
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WB8YYY
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« Reply #7 on: September 12, 2002, 12:43:21 PM »

You will probably be ok using a 40 meter center fed antenna without an antenna tuner.  If you have ever put up one before, you know the match is sensitive to environment.  But if you build one, put it up similar to how it will be installed at the field site, and tune it, i suspect it will be close enough at the field site.

Since you are only operating on one band, you can thus do without a tuner.  However, if you decide you want a portable tuner - check out the EMTECH ZM2 kit.  It is a fine QRP tuner and price is reasonable.  This one uses an LED as a VSWR indicator and is compact.  If you really want to scrounge for parts their are tuner designs in ARRL literature, and probably on the web.  
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WB8YYY
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« Reply #8 on: September 12, 2002, 12:48:00 PM »

You will probably be ok using a 40 meter center fed antenna without an antenna tuner.  If you have ever put up one before, you know the match is sensitive to environment.  But if you build one, put it up similar to how it will be installed at the field site, and tune it, i suspect it will be close enough at the field site.

Since you are only operating on one band, you can thus do without a tuner.  However, if you decide you want a portable tuner - check out the EMTECH ZM2 kit.  It is a fine QRP tuner and price is reasonable.  This one uses an LED as a VSWR indicator and is compact.  If you really want to scrounge for parts their are tuner designs in ARRL literature, and probably on the web.  
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