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1-10 of 13 messages
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Homemade HF vertical antenna help needed
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by KD7QLU on February 18, 2008
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I recently tried to make a homemade vertical HF antenna using instructions I found on the internet. The antenna consists of 30' of #14 wire wrapped around a 4' pvc pipe with a 30" length of wire sticking up from the top. At the base, the center conductor of the feedline coax goes to the wire. The shield of the coax is not connected to anything at this point, but should have radials connected to it as my counterpoise. I put a pl259 on the other end and connected to the back of my Kenwood TS570G. Here's the problem... When the center pin of the pl259 contacts the center of the antenna connection on the 570 I'm able to receive. As soon as I tighten it down and the collar of the pl259 contacts the outside threads of the connector, I lose reception. Is this a problem with the pl259 (screw on style from Radio Shack) or is this because I have no radials attached at this point? Thanks... Mike, KD7QLU
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RE: Homemade HF vertical antenna help needed
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by AE6RO on February 18, 2008
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Probably one of your PL-259s is shorted, or the coax center conductor is shorted to the braid. Best guess is the PL-259 at the radio.
What band is the antenna for? 30 feet is around a half wave on 14 MHz so I'm guessing 20 meters. Grounding might be a problem due to the very low input impedance of your antenna. Also, you need to match this low impedance to your rig's 50 ohms.
C'iao and 73, John
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RE: Homemade HF vertical antenna help needed
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by KD7QLU on February 18, 2008
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The antenna is for 20 meters. I'm trying to learn by making my own antennas. I checked continuity between the center conductor and the collar of the pl259 suspecting a short but there was no continuity. Also, I took the pl259 off, placed the center conductor into the radio and then touched the braid to the outer threads of the connection and when I did this I lost the signal. It was my understanding that the center conductor in the pl259 is isolated from the braid, and that the braid should have electrical contact with the outer coupling of the pl259. In this way the center conductor feeds the signal into the radio while the braid, connected to the pl259 coupling, grounds out to the outside threads of the radio's antenna connection. Do I understand this correctly? If so, it looks like the connection was correct which leaves only the lack of radials, but I'm not sure that would cause a total lack of reception.
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RE: Homemade HF vertical antenna help needed
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by WB2WIK on February 18, 2008
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The problem is you didn't connect radials.
Without radials, you have a high impedance load which is being effectively shorted out by the capacitance of the coaxial cable -- this doesn't work.
Connect a good set of radials to the system, complete the circuit, and it will work fine.
A vertical without radials fed by coax can be a short circuit for RF (not for DC, so don't expect to see the short on an Ohmmeter).
WB2WIK/6
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RE: Homemade HF vertical antenna help needed
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by KD7QLU on February 18, 2008
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Thanks... I ran out of time for the radials and then came down with the flu/cold bug that's going around. I'll get that done soon and see what happens.
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RE: Homemade HF vertical antenna help needed
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by WA4DOU on February 18, 2008
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Mike, the trouble is really an indication of just how poor this antenna project is, in a real and practical sense. When you plug in the center conductor of the coax, you're using the length of coax and the antenna on the end of it as a receiving antenna. When you make the shield connection to your rig, you are using only the antenna on the end of the coax as your antenna. If I understood your antenna correctly, it is a shortened, helically wound vertical. It probably has a radiation resistance of 5-10 ohms if you had radials in place. It would need a matching network or a 6:1 or 8:1 unun to match it to coax. A quarterwave vertical on 20 meters is only 17 feet long and much superior to this thing. Consider it a lesson in the fact that there are no free lunches and short antennas are often very deficient in performance. Yes, you could add an unun and make it work some, but why? Much cheaper to make a full quarterwave vertical. Take the wire you wound it with and make radials.
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RE: Homemade HF vertical antenna help needed
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by WB6BYU on February 18, 2008
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When you only connect the center pin of the coax, the entire length of
coax is acting as an antenna. This is considerably longer (I presume)
than the antenna connected at the other end, so of course it picks up
more signal.
When you ground the coax shield the cable acts as a feedline rather
than as a single wire antenna. Since the antenna at the other end is
smaller (and probably far from resonance) there is less signal heard.
There is no magic length of wire that will make a vertical antenna
resonant on a particular band when wound into a coil - you can say
that a particular antenna requires a particular value of inductance, but
the amount of wire required to get that inductance will vary with the
diameter of the coil, turns spacing, length of the coil form, etc.
But in any case, you need some quarter wave radial wires on the ground
side of your antenna at the feedpoint to provide the other half of the
antenna (connected to the coax shield.) And it will take some adjustment
to get it resonant on 20m: you'll have to adjust either the inductance
of the coil or the length of the wire above the coil to get the resonant
frequency where you want it. An SWR analyzer or at least a dip meter
will be much easier than trying to measure the SWR using your
transmitter.
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RE: Homemade HF vertical antenna help needed
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by K4SAV on February 18, 2008
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I am interested in seeing a link to that antenna you found on the internet. You didn't say how long the PVC pipe was, so I don't know if the coil is wound tightly or spread out over a long length. I did a quick look at it, and if the coil is closely wound, the antenna should resonate at 144 MHz (that's 144 MHz, not 14.4 MHz). The coil would be operating above its resonant frequency, and would look like a capacitor, not an inductor. So assuming that is not what it is supposed to do, where did you find this antenna?
Jerry, K4SAV
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RE: Homemade HF vertical antenna help needed
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by KB3PNE on February 18, 2008
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that screw on style pl259 from rat shack sucks big time, you will never get it to contact anything for more than 10 mins, throw it, they do do a better one if you look in the store, i got the one with the longer body, its a solder type and is pretty good.
kb3pne, colin
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RE: Homemade HF vertical antenna help needed
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by WA3SKN on February 19, 2008
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You can check for a short with an ohmmeter... but the problem is probably the lack of radials! At 30 pf a foot, that coax feed is nothing but a capacitor... shorting any RF to ground before it gets to the receiver.
Finish the project, ADD THE RADIALS!
73s
-Mike.
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