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Need More R
- Thread starter C2
- Start date
I still would like to hear why you're seeking to change anything if the antenna is already resonant? Just wondering
Resonant is resonant, and 50 ohms is 50 ohms, but 50 ohms does not necessarily mean resonant. They are two differant things.Granted a 1.6:1 SWR (30 ohms) is not bad but perhaps it is if running high power or when changing freq very far from the resonant point. Providing a perfect 50 ohm match will allow more "wiggle room" for either case.
Because it is resonant at too low an R value.
R=30, X=0, SWR = 1.67
The impedance curve is exactly where I want it except for it being a bit low, so the 2:1 bandwidth points are shifted away from where I need them. If R was 50, it would be perfect! So I would like to add 20 Ohms...
It was the classic design in the ARRL Antenna book for a 2m 5/8 wave, Which I would actually model as:
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|-(G)-////-(+)-///----------<|
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Doc, is that not one loading coil (which I call series inductance) with one more coil to ground (shunt inductor)?
After fiddling with it for a while and determining, while I could get it to load nicely with good match, that my build exhibited about 3 dBd loss, I went with a simpler design:
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(G) (+)-///----------<|
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The simpler design also loads up nicely with fairly good match, and exhibits about +3 dBd.
But the question is more generic, how to add more R without affecting X?
R=30, X=0, SWR = 1.67
The impedance curve is exactly where I want it except for it being a bit low, so the 2:1 bandwidth points are shifted away from where I need them. If R was 50, it would be perfect! So I would like to add 20 Ohms...
It was the classic design in the ARRL Antenna book for a 2m 5/8 wave, Which I would actually model as:
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|
|
|-(G)-////-(+)-///----------<|
|
|
|
|
Doc, is that not one loading coil (which I call series inductance) with one more coil to ground (shunt inductor)?
After fiddling with it for a while and determining, while I could get it to load nicely with good match, that my build exhibited about 3 dBd loss, I went with a simpler design:
|
|
|
|
(G) (+)-///----------<|
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|
|
|
The simpler design also loads up nicely with fairly good match, and exhibits about +3 dBd.
But the question is more generic, how to add more R without affecting X?
"insulate" is the wrong word....what I was trying to say is that you can't just have a solid conductive shaft and wrap the coil around it. The coil has to be an open coil and if it's not strong enough to support itself you have to form it around something non-conductive like fiberglass...
"I have an antenna...."
if you want to increase R some more detailed information about the antenna in question would be helpful....
is it a 2m 5/8?
if you want to increase R some more detailed information about the antenna in question would be helpful....
is it a 2m 5/8?
OK, it is the classic 2m 5/8 from the ARRL antenna handbook, but made a bit differently.
See pics...
First design:
I made a groundplane with two brass rods through PVC. Then I coiled some #12 wire around the PVC, then into the PVC through a hole and out the top. The center tap comes through the center of the PVC and coil, and then out a hole to a tap point. I monkeyed around with the tap point and overall length until I got a good match.
Second design:
I made a groundplane with two brass rods through PVC. Then I coiled some #12 wire inside the PVC, then straight out the top. I monkeyed around with the number of coil turns and overall length of the wire until I got a good match.
I actually measured about 6 dB improvement with the simpler, second design. The first is worse than a simple direct coax fed, vertical dipole, the second design is about 3 dB better than that dipole...
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I am using a Signal Generator to feed a dipole antenna with the feed point about 10 feet above ambient soil level, output power ~ 0 dBm.
My antenna-under-test feed point is also 10 feet above ambient soil level and 50 feet away, horizontally. This antenna feeds a Spectrum Analyzer with a reference level set to ~ -50 dBm and 2 dB/division, with RBW to 10 kHz and VBW to 1 kHz.
First I take a reference measurement at the antenna-under-test site using an identical dipole (two dipoles). Then I swap the receive antenna to either of the 5/8 wave antennas I've built and measure the change in received signal strength. Note that the antenna-under-test feeds the spectrum analyzer at a point approximately 20 feet perpendicular to the midpoint between the two antennas. All tests are conducted outside in an open field.
Of course this test does not really take into account the radiation angle, but I am just looking at receive strength. I plan to make measurements on the transmit side when I have more time, taking the identical measurement and two more measurements with the reference dipole (for receive) at additional heights of 25 and 60 feet. This will give me 0*, 27* and 50* elevation measurements.
It looks like the attached...
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I did not say ambient soil any more than one would say ambient air. I said ambient soil level, much like someone would say ambient air temperature.
That is an intersting idea. I can bend the radials down and measure the net affect. I'm not sure if that will improve the receive, however...
I took another look at that 'second' antenna. What it amounts to is a loaded antenna with a 'stinger' 5/8 wave length long. No idea what what the equivalent antenna 'length' would be, but if you end up with a 50 ohm input, it's definitely not a 5/8 wave anymore. that might explain the difference you're seeing between the 5/8 and it. You think?
- 'Doc
- 'Doc
I've thought about it and the only difference is the missing paralles inductor, which I could add across the coax. Maybe?
I think the next size up is a 3/4 wave? That's not what I want!
I made some Smith plots of three antennas with a sweep from 142 MHz to 150 MHz. Markers at 144, 146, and 148 MHz counting clockwise for all plots...
The first is a straight vertical dipole direct fed with coax. This is a simple design of 12ga wire soldered to a BNC and hung from the hot end with the coax comming off at 90 degrees for 1 wavelength. It's hung indoors in the center of a room about 7 feet off the ground.
The second is the same antenna with the shield leg bent at 90 degrees, or an L dipole.
The 3rd is my now mystery antenna which is supposedly a 5/8 wave vertical element with a loading coil at the base and 4 ground radials (as pictured earlier).
The yellow dotted circle is the 2:1 VSWR circle. Anything inside is less than 2:1. The center at 1 is an ideal resistive 50-Ohm load.
Here is a bit more info about a Smith chart:
http://www.eetasia.com/ARTICLES/2008FEB/C/EEOL_2008FEB18_RFD_NETD_NT4.jpg
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