I have a old cobra magnetic mount antenna. I was thinking using steel whip and coax to make 2m 7cm dipole. Have some you guys done this before? I have read this thread and saw the negative post. But am talking about using it to make a homebrew antenna. Any suggestions work be appreciated.
Making a 2m/70cm dipole
- Thread starter km4mcm
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wb6uqa
Member
2 meter antenna
If you want a 2 meter buy or make one. Cut five 19 inch coat hangers, get a male pl 259 and make a ground plane.. One in the top and 4 radials in the other holes at 45 degrees.
If you want a 2 meter buy or make one. Cut five 19 inch coat hangers, get a male pl 259 and make a ground plane.. One in the top and 4 radials in the other holes at 45 degrees.
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N3JI
Member
For the dipole, cut it at 20" each side and measure it. It should work as a 3/4 wave on 440. What exactly do you want to know?
N3JI
Member
Check into J-poles or coaxial collinear antennas. They are fun to build and work very well. A single element J-pole is very easy to build, coaxial collinears are bit more of a challenge, but have good gain. I built one for 800 MHz (which was a bit harder due to how critical the measurements are), and it works extremely well.
paulears
Member
I'm actually a lover of straight dipoles, not the folded ones. I'm great at electronics and but making a decent looking one, rather than a bodge is a chore - so I buy cheap FM broadcast dipoles cut for the 100MHz band, and then shorten them for VHF. A short length of cable from the terminals to an N type - my preferred aerial connector, and away you go for a very low outlay. I've found ¼ waves cut for VHF are not bad at all for UHF, being a good match to the transmitter. I've fiddled with J-Poles, and the popular UK Slim Jim pioneered by Fred Judd, G2BCX in the seventies. He lived quite near me, and loads of the local hams swore by these aerials back then, mainly, I think, because the angle of strangest signal was narrower, not wasting so much going up like dipoles. As Joe said, they're quite easy to fiddle with and with a distant repeater for a constant signal strength, you can easily compare them, and when you build one with the most gain, you can then build a stronger version. Loads of people connect the conductors to the antenna with clips, or twists, then fiddle them up and down until you get a match, and then you build another with permanent connections.
N3JI
Member
I assume you mean 1/4 wave per leg, or a 1/2 wave dipole. For folded 1/2 wave dipoles, the impedance is around 300 ohms, so some type of transformer balun would be needed to match to coax. Just mentioning this since directly connecting a folded dipole to a rig with 50 ohm coax might not work as expected.
paulears
Member
Yep - straight dipoles, not folded ones which require a balun. I did mention NOT the folded types.
wb6uqa
Member
2 meter dipole
A balun not needed on 2 meters or 450 mhz. I would go to a radio store and match your mount to a dual band antenna..Be careful of polarity , cross polarity could be a 20 db loss.
A balun not needed on 2 meters or 450 mhz. I would go to a radio store and match your mount to a dual band antenna..Be careful of polarity , cross polarity could be a 20 db loss.
paulears
Member
I think you may need to revisit that - baluns are needed at ANY frequency where you need to convert a balanced antenna feedpoint to an unbalanced feeder, and even more important when you also do the impedance conversion - so on a folded dipole, getting the impedance down to close to 50 Ohms is pretty important.
What is this mount you mention? He wants to make a dipole. Dipoles are already dual band if you consider that a ¼ wave dipole on VHF is also a ¾ wave dipole on UHF ham bands.
He wants to make something - and a dipole is a fairly simple thing to produce, yet works rather well. I assumed his intention was to avoid buying something and having an experiment. Cross polarisation (not polarity, which is a totally different phenomenon) can actually reduce reception strength to zero, or close to it.
What is this mount you mention? He wants to make a dipole. Dipoles are already dual band if you consider that a ¼ wave dipole on VHF is also a ¾ wave dipole on UHF ham bands.
He wants to make something - and a dipole is a fairly simple thing to produce, yet works rather well. I assumed his intention was to avoid buying something and having an experiment. Cross polarisation (not polarity, which is a totally different phenomenon) can actually reduce reception strength to zero, or close to it.
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