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ryan
| | Posted on Saturday, December 21, 2002 - 4:50 pm: | 
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i got a vise grip mount for my 1/4~ figerglass whip. if i mounted it up high and made a coil choke 1/4~ down the coax, would i get a decent SWR? if im thinking correctly, the last 9' or so of coax will radiate and basically work like a dipole |
bruce
| | Posted on Saturday, December 21, 2002 - 6:02 pm: |
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yep |
bullet
| | Posted on Saturday, December 21, 2002 - 6:55 pm: |
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i believe it will. (cfr) |
ryan
| | Posted on Sunday, December 22, 2002 - 12:38 am: |
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sweet. i cant wait to get this thing up and on the air. i wanna throw some waves |
ryan
| | Posted on Sunday, December 22, 2002 - 4:23 pm: |
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ok, i got this rig up and i only got it down to a 1:2. im using RG-213 coax. i got a coil around 7" in diameter and am rolling it up and down the pole to try to get the swr right. |
bruce
| | Posted on Sunday, December 22, 2002 - 10:40 pm: |
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what coil are we talking about the coax coiled up or a coil arould the coax myself i would use several beads at the choke point or just run a peace of sheild back down the coax from the feed for 102 inches |
ryan
| | Posted on Sunday, December 22, 2002 - 10:58 pm: |
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its a coax coil. im thinking of switching over to a thinner coax than the 213 so it will be easier to wind it. |
bruce
| | Posted on Monday, December 23, 2002 - 4:45 am: |
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look in the antenna handbook for ferite bead chokes i use them on VHF they make beads big enough to slip over the coax and you can move them up and down but a even easer way is just to fold the sheild back and shrink tube over it. My MFJ used 20 turns of rg-58 on a 6 inch form for a rf choke it worked well |
Highlander
| | Posted on Monday, December 23, 2002 - 11:57 am: |
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1.2 sounds pretty darn good to me. I'd start talking on it! |
ryan
| | Posted on Monday, December 23, 2002 - 3:38 pm: |
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i want to get it below 1:1.5 if i start talkin. im using some older cbs like the cybernet gems |
Tech833
| | Posted on Tuesday, December 24, 2002 - 1:08 am: |
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A coax coil below a 1/4 wave whip will not make the setup act like a 1/2 wave dipole. You would need to add a sleeve over the coax cable in order to transform it into a coaxial dipole. All you have done is created a coax shield choke. That does basically nothing in this case.
Now, I'm not saying this is bad, I'm just saying you will not get the performance you would with a coaxial sleeve dipole. |
ss8541
| | Posted on Tuesday, December 24, 2002 - 3:10 pm: |
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ryan, according to all the info that i have here, the coaxial coil should work just fine, and actually work better than a coaxial sleeve to make a 1/2w vertical dipole.
according to the rfworks company there are 4 types of current on an unbalanced(coax) antenna system.
I1 is the center conductor current
I2 is current on inside of coax braid
I3 is the current on the leg of the antenna that
attaches to the coax braid(I2 becomes I3)
I4 is the current on the outer braid of the coax.
it comes from the part of I2 that didn't turn
into I3 current
if there is no leg of the dipole(I3), the current of I2 does a complete 180 degree turn and becomes I4 current(outer braid). the outerbraid is now the I3 leg of the antenna and is radiating. this is exactly what you are doing here ryan. the antenna is your I1 and the outer braid of the coax has become your I3. placing a coil a 1/4w down chokes this current(I3/I4) and lets the 1/4w section of the outer braid act as resonant counterpoise.
this was proven in an article in the august 1999 issue of CQ by rolf brevig(LA1IC). he made what he called 'feedline verticals' for 2 and 6 meters. there was no coaxial sleeve. just coax using the center conductor as the vertical radiating element and the braid as the counterpoise. the braid was just cut off clean. he called this the point of departure for the I2 current(where it did a complete 180 and was now radiating on the outer braid as part of the antenna system - counterpoise). LA1IC, also said that with the coaxial sleeve dipole he was unable to get swr below 2:1. mainly because of capacitance between the sleeve and outer braid and because there was nothing to tell the currents on the sleeve to not go back down the outer braid of the coax. with the choke and no sleeve method he said he could get swr's of close to 1.1:1 for both the 2 and 6 meter antennas. he also claims that this method makes the antenna more broadbanded than the coaxial sleeve method.
bascially what you are doing here is the same thing that LA1IC did. the difference is that you are using an actual antenna as the radiator where he used the center conductor of the coax. i would recommend using ferrite cores as your choke. you can place them over the coax and they will 'choke' the rf on the outer braid much better than the coil. a coil works good to choke rf if you have it wound -just right- for the freq you want to choke, and this is why i recommend using ferrite. radio shacks snap together chokes or material 43 or 75 will all work good to choke these currents greatly. |
ryan
| | Posted on Tuesday, December 24, 2002 - 4:57 pm: |
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sweet, ill have to go out and purchase a bunch of ferrites. my coil isnt the best one figuring im using thick rg-213 now so once i step down to RG-8, ill be able to wind a better coil. ill try a couple of different things and let you all know how it works |
Tech833
| | Posted on Tuesday, December 24, 2002 - 8:40 pm: |
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Currents cannot flow different directions at the same time over the same conductor without cancellation. I wasn't saying it wouldn't work, I was just saying that it won't work as well as a real sleeve dipole.
As for the high SWR, there was obviously something else wrong that the author had not considered.
I encourage experimentation, and applaud people who do. I only give the advice from my own 20+ years experience as a commercial engineer.
FYI: Sometimes the hams get it a little wrong and need to be corrected. Their heart is in the right place, but they sometimes lack the education and theory. Don't take information in a ham magazine article as gospel truth. Remember, most of the hams writing those articles are not 'experts', but experimenters just like you, and sometimes, their conclusions are biased and prejudiced by their investment of time in the project. Find out for yourself. |
ss8541
| | Posted on Thursday, December 26, 2002 - 11:50 pm: |
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ryan, i agree with 833, you need to experiment.
due to the 'skin effect'(rf travels on the skin of a conductor), the coaxial braid should act like 2 separate conductors. the inner part of the braid as one conductor and the outer part of the braid as a 2nd conductor. if this -is- the case(which i have found to be so in my work in business band radio and amateur/cb radio), and there is no counterpoise leg of the antenna(I3 current), the inner part of the braid should carry I2 current while the outer part of the braid -should- all be I3 current. this means that the outer braid -should- become the couterpoise for the vertical dipole and will radiate with no to minimum cancellation from the I2 current.
833 has brought some valid points to disagree with this, so i am very interested in reading your findings.
i am also not saying 833 is wrong. he has many more years experience at this than i do. but from what i have learned(from text books/experince) in my short time at rf professionally i -feel- that this has a very good -chance- at working well. then again, maybe it will not.
here is a link to the radio works website that will show you better what i am talking about for I1-I4 currents; http://www.radioworks.com/nbalun.html on this page click on 'feedline isolation'. if you take away what -would- be I3(that would be counterpoise), then I4 -should- become I3 plus what I4 is already(outer braid radiating/counterpoise). |
ss8541
| | Posted on Thursday, December 26, 2002 - 11:55 pm: |
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also, the radio works site does not say that I3 will become I4 if there is no coutnerpoise. i read this into that, and this should be stated for future reference(in case someone tries to make it look like i said it stated that). this is also why i used -should- in so many places in my last post. |
bullet
| | Posted on Friday, December 27, 2002 - 2:42 am: |
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ryan,
also look up controlled feedline radiation!
it works..... |
bruce
| | Posted on Friday, December 27, 2002 - 7:27 am: |
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i still say fold back the brade over its self and shrink rap over it
Bruce |
ryan
| | Posted on Friday, December 27, 2002 - 7:13 pm: |
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since i was real sick today, i didnt get out to get ferrite beads. i want to make this antenna easily deployable for when i go camping or something. i also want to make it look better than one of those stripped coax antennas. i should be able to get pics up when im done |
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