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Forum Index : Microcontroller and PC projects : Small J-pole?
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Grogster Admin Group  Joined: 31/12/2012 Location: New ZealandPosts: 9610 |
Hi all.  I normally use this J-pole calculator to work out an aerial's dimensions, and the J-pole works very well at FM frequencies. However, this time, I have a need for something J-pole-ish, but it needs to be much smaller, and still be a match to the transmitter. Frequency will be 107.7MHz @ 6W or so, therefore using a simple whip is not really any good, as you need a decent match once you run a few watts, or the bad SWR will kill the final. As I say, the J-pole is really my go-to aerial for FM work, but it will be too big, as I need to mount it on top of a vehicle. The transmitter is used with a microphone on a pilot vehicle for farm tours. A big half-wave J-pole will look ugly on top of a vehicle, but it will also flop around too much due to it's height. I am wondering if I can just calculate for half-wave as per the link above, but then half the dimensions, so the radiator would be quarter-wave, and the matching section would be eighth-wave? Possible, or am I totally wrong with my assumptions on halving the dimensions? I expect I probably am, as half does not mean it will still be a match at 107.7MHz. Quite happy to look at any alternative, so long as it is small, and can be matched to the 5-6W output. EDIT: The same site also has details on the old faithful GP antenna here. This is still a bit on the large size at 107.7MHz, but I could work with this size if I had to. Certainly shorter then the J-pole at the same frequency. I am wondering if I can make use of the vehicle roof as a ground-plane? The problem is that probably still would not be a match without tuning the radiator depending on what vehicle it was fitted to(different size roof's). The GP ant on a pole fixed to the vehicle is probably the best way forward, but happy to consider alternative suggestions. Smoke makes things work. When the smoke gets out, it stops! |
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| Boppa Guru  Joined: 08/11/2016 Location: AustraliaPosts: 814 |
A vertical whip cut to frequency will be fine, its what every cb/amateur radio that goes mobile uses, I use 100w into a tuned whip with no issues (and thats on a radio that has to deal with entire bands- not just a single frequency!) have you got a vswr meter?- if so just tune a whip until its good to go |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9610 |
Yes, I have a couple of different SWR meters. Are your CB whips using the vehicle roof as the GP? EDIT: Wavelength of 107.7MHz is 2783.5mm. Divided by four equals 695.87mm, so a tapered stainless-steel whip cut to about 695mm should be about right(for quarter-wave) - is that what you are saying? Smoke makes things work. When the smoke gets out, it stops! |
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| Boppa Guru Joined: 08/11/2016 Location: AustraliaPosts: 814 |
Using a straight wire antenna as the antenna (as opposed to a helical whip like many cb style whips where the wire is wound as a coil to shorten the physical length) it would be 66.2cm tall, and would require a metal groundplane of 132.5 x 132.5cm below it |
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| Boppa Guru Joined: 08/11/2016 Location: AustraliaPosts: 814 |
I'd see if I could find a stainless whip, start slightly longer and just trim for min vswer then |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9610 |
I have an 88-118MHz 50-ohm tapered stainless whip I can cut to the correct length(plus a bit more so I can tune it up and down), and fitted to a magnetic base, the vehicle roof should act as the GP. That's the theory for the moment, yes? Smoke makes things work. When the smoke gets out, it stops! |
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| Boppa Guru Joined: 08/11/2016 Location: AustraliaPosts: 814 |
Ive seen a few hams that have used half sized jpoles as tx, but they do tend to have fairly high vswr, not an issue if you have a matcher, but you obviously wouldnt want to have one just for that purpose. Many uhf stainless whips have a ferrule (the screw bit at the bottom) that has a small allen key to hold the whip in (which is just a piece of stainless rod usually) if you cant find a stainless long enough, just get one of those and a length of rod and make your own one long enough |
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| Boppa Guru Joined: 08/11/2016 Location: AustraliaPosts: 814 |
Id avoid the mag base if possible and just fit a antenna base to the roof (if you can drill holes in it that is)- magnet bases are notorious for high vswr what type of car is it? (other option might be a set of roof racks electrically bonded to the car body, with a square of checkerplate, or ply with gal on it It all depends on how intolerant the tx really is, most can run up to 3:1 without damage, just reduced power |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9610 |
Yeah, that is the problem. I don't know what kind of vehicle it will be going on, but I fully expect it to a farm ute. I will ask to see if I can get an idea. They only want it for when they are doing farm tours, so permanent mounting on the roof is most definitely out. That was why I was thinking magnetic base, but I hear you on the bad VWSR that can produce. I will see if I can find a photo I did of a whip with magnetic base I made out of a lolly tin(for the base), and some old hard-drive magnets. Very thin foam-rubber glued to the bottom of the tin, and you just 'Stuck' it on the roof. Very effective. Tin was connected to shield. With the thinnest foam or felt you can get to prevent the tin scratching the paint, I expect there was some kind of capacitive coupling between the tin and the roof - that might have been what was upsetting the SWR. Smoke makes things work. When the smoke gets out, it stops! |
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| Boppa Guru Joined: 08/11/2016 Location: AustraliaPosts: 814 |
Ive seen a fairly effective mag base setup homemade by a ham, had a dipole mount attached to 4 wires, each with magnet on the end and the dipole mount had a magnet on it too- put whip in center and stretch the 4 wires out and stick them to the roof as well, he was getting very good results If it has to removable/portable- Id say just use the magnet base and trim for best results and be done with it, the tx shouldnt be that touchy that it wouldnt work, and its not as if you want an excessive range, you arent chasing skip lol, 6w should get you several km!!! I doubt a tour group would be THAT spread out! |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9610 |
Totally agree. I just get a little paranoid about correct match once up to 1W or more. Less then a watt, and you can just dangle a bit of wire of just about any length, as there is not enough power to reflect back to cause any damage.Higher power transmitters do NOT like mismatched antennas on their output.  ....and an FM transmitter is a 100% duty-cycle device, so the match IS important if you want the final to survive. CB radios can probably get away with a bad match, as the duty cycle is generally never more then 50/50 - if that. In any event, the 'Off-time' allows the final to cool off if it is a bad match at transmit.  Smoke makes things work. When the smoke gets out, it stops! |
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| Boppa Guru Joined: 08/11/2016 Location: AustraliaPosts: 814 |
Yeah I got an fm 2m rig that was occasionally used for data work so extended tx times, its a 25w unit, but being a commercial rig it is fairly tolerant of mismatches- Ive alway tried for best coupling tho- simply to get as much out of the rigs as possible (many moons ago when I started out in 27 megs, I built a 7 element cubical quad for it- now THAT had some punch- but luckily I was on acreage as it was basically the width, height and length of a bus- up 100ft on a tower made out of an old jib crane lol- not something I think I could have gotten away with in suburbia!) |
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TassyJim Guru Joined: 07/08/2011 Location: AustraliaPosts: 6283 |
I regularly use quarter wave verticals with a magnetic base for 144MHz and would not hesitate to use one for 107MHz With a magnetic base you can position the antenna close to the center of the roof, not that it is too critical. There is enough capacitance between the magnet and the roof to get the ground-plane working. Use one of the larger magnetic bases and not the mini ones you get with the TV tuners etc. A thin sheet of plastic or cloth between the base and the roof will help reduce any marking which tends to upset the missus. They do make a racket when you go under low hanging branches. Jim VK7JH MMedit |
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