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off center fed vertical dipole for scanner

 
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sduckworth
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Joined: 19 Mar 2009
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PostPosted: Thu Mar 19, 2009 7:02 pm    Post subject: off center fed vertical dipole for scanner Reply with quote

I've been building some off center fed vertical dipoles, using 1/2 " copper tubing and schedule 40 pvc for spacers/connections. Feedpoint impedance should be 300 ohms for a 4:1 balun (the cheap kind, available everywhere) and a 75 ohm coax feedline (RG-6 QS). My frequency focus is fairly wideband, but mainly 150-170 Mhz with some interest in the civilian aircraft band (118 Mhz to 137 Mhz) and the military aircraft band ( about 240-350+ Mhz). I've seen at least 3 different sets of formulas to attain the correct offset for 300 ohms, and this has left me confused. I also built the 18": 48" homebrew scanner antenna touted in the Radio Reference Wiki, but it resonates at about 468/5.5 = 85 Mhz, at the lower end of the FM broadcast band--to me, not ideal for scanners. Which formula and dimensions would you recommend for 150 Mhz, say?

I've also been considering connecting this off center fed vertical together with a RS ground plane antenna (no. 20-176, the $26 one for scanners) via a regular TV combiner/splitter or a co-phase cable like that used with dual CB antennas. These two different antennas would be at opposite ends of a 1" pvc support boom (what about spacing?). I suppose there may a phasing problem, but what if their resonant freq is the same?
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pete463251
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PostPosted: Thu Mar 19, 2009 9:41 pm    Post subject: Some thoughts.... Reply with quote

Some thoughts....

1) You could try a folded dipole antenna, which have an impedance that is roughly 300 Ohms, and are easy to construct.
http://www.antenna-theory.com/antennas/foldeddipole.php

2) If you have a schematic for an antenna that resonates at 85 MHz, you can scale all the dimensions by 0.5 (half the size) and it would resonate at 170 MHz. In general, you can figure out what the new size would be to achieve your desired resonant frequency simply by scaling the original model. One place that discusses this is:
http://www.antenna-theory.com/measurements/scalemodel.php


If you do the multiple antenna approach, if the antennas radiate in the horizontal plane primarily and are spaced vertically (one above the other), the spacing will cause virtually no impact on the horizontal gain measurements, but will affect the radiation for directions away from the horizon.

Also, in general antennas can be made more wideband by making them fatter. That is, if you are using dipoles, use fat wire or wrap it in aluminum foil, and that should increase the bandwidth.
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sduckworth
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PostPosted: Fri Mar 20, 2009 1:11 am    Post subject: Reply with quote

I've built a bunch of folded dipoles from twinlead, but never oriented one vertically, though I suppose that's feasible. Want something to mount on a mast, vertically polarized, about 20 ft off the ground, will handle some wind velocity, and cover 118-137, 140-170, and some military air/ space freqs (230 Mhz +), maybe even some 460 Mhz. I know I'm asking for a lot.

I'd also like to "co-phase" this off center fed dipole or other, more appropriate, but most importantly, cheap alternative with a RS 20-174 groundplane and connect the two to a Radio Shack Pro-2006 scanner. I'd like to avoid diplexers, even cable switches, if possible. That's why I asked about simple TV splitter/combiners (I know--3 db loss for each insertion).

What about using 1 1/2 to 2 inch copper tubing? Would that increase the bandwidth that much? Worth the trouble and weight?

Are my goals feasible?
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pete463251
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PostPosted: Fri Mar 20, 2009 6:14 pm    Post subject: Blade Antenna? Reply with quote

Its not super unreasonable of a goal, but its not easy either.

For instance, here is a blad antenna (basically a dipole antenna that is elongated so it is shaped like a paddle):
http://www.trivec.com/Application.aspx?c=Style&v=1

The frequency range specified is 30-400 MHz, which is a bandwidth greater than 10:1. However, really it operates around 120 MHz - 360 MHz I would guess, given how low the gain is in the low-frequency region.

You could try to build a blade, which is fed the same way as a dipole is. This might get you the bandwidth you need. The wideness of the blade is analogous to making a dipole "fatter", so increasing the thickness can have some extraordinary effects. You'll have to play around and see what you can do.
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kuku202
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Joined: 10 Feb 2015
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PostPosted: Tue Feb 10, 2015 6:30 am    Post subject: Reply with quote

However, if you are mounting antennas indoors, you will always get a lot of reflections off the wall, ceilings, floor, room objects, etc... So the roof won't be a huge problem (you could use the roof as aground plane if you mounted the antenna upside down on the roof, but you probably don't need to do that).


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