Close proximity to objects usually lowers the feed point impedance of an antenna. That 75 ohm feedpoint might even go close to 50 ohms, making the SWR close to 1:1.
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Can someone help me tune my homemade dipole?
- Thread starter mofugly13
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Close proximity to objects usually lowers the feed point impedance of an antenna. That 75 ohm feedpoint might even go close to 50 ohms, making the SWR close to 1:1.
^^^^^ Complete twit when it comes to antennas, but I do remember something about metallurgy that a tech here might elaborate on.
Stainless Steel and copper has a different.... resonance? When you put a 102 on a car, you use the spring to make it a 108" from the stud. If you mount a fiberglass with a copper core you leave the spring out... or something like that.
I'm thinking that you have a lop eared rabbit here. The signal coming from each acting slightly different. There is surly a harmonic between the two that could cause reflection and prevent the SWR from falling below 3:1. IF you were to leave the copper pipe in there, you might need to trim it to a shorter or longer length (I remember watching YouTubes on this 102" whip thing). Personally I like the idea of having twin stainless whips. It's only $25 more to your investments and will eliminate any harmonics between the steel and copper.
<Antenna Twit will now sit down and return the meeting back over to people who know what they are doing >
Stainless Steel and copper has a different.... resonance? When you put a 102 on a car, you use the spring to make it a 108" from the stud. If you mount a fiberglass with a copper core you leave the spring out... or something like that.
I'm thinking that you have a lop eared rabbit here. The signal coming from each acting slightly different. There is surly a harmonic between the two that could cause reflection and prevent the SWR from falling below 3:1. IF you were to leave the copper pipe in there, you might need to trim it to a shorter or longer length (I remember watching YouTubes on this 102" whip thing). Personally I like the idea of having twin stainless whips. It's only $25 more to your investments and will eliminate any harmonics between the steel and copper.
<Antenna Twit will now sit down and return the meeting back over to people who know what they are doing >
AC9KH
Member
Just for reference sake, an antenna tuner does fix the real problem. It is an LC network. So it matches the transmitter's impedance to the feedline. The real problem is that a dipole is a balanced antenna so it needs a balanced feedline if it is not at resonance. With the antenna at resonance, the capacitance and inductance values of the antenna are equal and you are dealing with pure resistive values.
Coax is not a balanced feedline. If you do buy a tuner, buy one with a balanced output and use air-insulated hardline instead of coax. Problem solved. It doesn't matter what the impedance mismatch is between the feedline and the antenna in that case because the losses on parallel feedline is so low as to be negligible. On a CB rig running only 4 watts on the carrier, you could probably get by with 300 ohm twinlead from the tuner to the antenna and it would work fine.
The biggest problem with that whole "solution" is that a good tuner costs more than the most expensive CB radios. And you don't really need a tuner because a tuner is only really needed when you intend to run a particular antenna on different frequency bands.
The biggest problem with the antenna in this thread is that it is a balanced antenna with an unbalanced feedline. So the easiest solution is turn the antenna into an unbalanced antenna by converting it to a ground plane type with the ground plane grounded. And then it will work fine.
prcguy
Member
Since the CB band is fairly narrow, a dipole has no problem operating at resonance and matching well to 50 ohm coax. Even though a dipole is considered balanced, if you feed it with coax all the power will be radiated and life goes on. No need to use air insulated hard line or buy a balanced output tuner.
Using air insulated hard line in a system that really needs a balanced tuner is not the right way to go anyway, you would use an actual balanced feedline made of parallel conductors.
prcguy
Using air insulated hard line in a system that really needs a balanced tuner is not the right way to go anyway, you would use an actual balanced feedline made of parallel conductors.
prcguy
AC9KH
Member
Yeah, sorry about that. I don't know what I was thinking there - I meant ladder line.
All I'm trying to say is that coax fed vertically polarized dipoles are not that common with HF because of the length of the antenna and sometimes the engineering challenges with preventing common mode current on the coax shield
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