mlhm5
Active Member
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Simple J Pole FM Antenna Plans
- Thread starter mlhm5
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Morden2004
Relaxin'
mlhm5 said:
I'm not certain but I don't think that this 'J' antenna will perform too well as a receiving antenna for FM. I think that this is a 3 meter base transmitting antenna. If I recall my antenna theory correctly, this antenna will propogate a vertically polarized waveform - and I'm guessing that as a receiving antenna it will be most suitable to the same polarization.
Again, I may be wrong. I'm digging way back deep into the recesses of a long-lost electronics and RF theory courses so perhaps someone with better skills will jump in here and correct me if I'm wrong.
Paul
Charivari
Fighting Naïve Realism
That first link is a good one and the book it came from is even better. I used to browse through an old copy looking for exotic, high-gain designs that I could possibly build to feed my old Hallicrafters for shortwave listening. It's actually a simpler design than the second you linked to because you can build it completely out of pipe fittings found in the local hardware store and be soldered together just using a plain ol' roll of plumbers solder, can of flux, and a propane torch. (Don't forget the sand paper.) I would recommend, though, to use at least 3/4" pipe or larger else you'll get some flex in high winds esp for the FM band. I built a two meter J-pole out of 1/2" copper water pipe and when the wind started gusting over 80 mph (just about a dozen times each winter), the flexing would cause the matching section to go out and in and out of tune as the distance shrunk and enlarged.
As to the concern of polarization, almost all modern FM stations are vertically polarized so a vertical like this will perform much better than any horizontal design. General rule of thumb, HF (1-30+ MHz) is horizontally polarized, VHF (50 MHz - ~1 GHz) is vertically polarized, and above that is circularly polarized, though VHF amateur satellites are circularly polarized as well.
- JP
As to the concern of polarization, almost all modern FM stations are vertically polarized so a vertical like this will perform much better than any horizontal design. General rule of thumb, HF (1-30+ MHz) is horizontally polarized, VHF (50 MHz - ~1 GHz) is vertically polarized, and above that is circularly polarized, though VHF amateur satellites are circularly polarized as well.
- JP
Morden2004
Relaxin'
JP: Thanks for the follow-up info. Perhaps I'll try that 'J' antenna just to see how it works here on the shore.
Paul
Paul
Morden2004
Relaxin'
I'm considering trying this antenna
But, I have a question about copper mixed with other metals.
IIRC, copper and steel are a poor combination because you get etching at the junction due to the galvanic corrosion with two dissimilar metals.
I was wondering if one can find aluminium tubes to use instead? Or, is there a way to avoid this problem if you use copper water pipe?
Paul
But, I have a question about copper mixed with other metals.
IIRC, copper and steel are a poor combination because you get etching at the junction due to the galvanic corrosion with two dissimilar metals.
I was wondering if one can find aluminium tubes to use instead? Or, is there a way to avoid this problem if you use copper water pipe?
Paul
For dissimilar metals in contact, one can impose a DC voltage equal and opposite the redox potential delta. Dissimilar metals in contact with a closed circuit make a battery. Push a voltage against that battery and no current flows and corrosion (from this source) is reduced. Or else, break the return path for the electrons.
Is it worth the hassle? Probably not.
Aluminum and iron have the same issue but aluminum rapidly develops a thin layer of ceramic Al2O3 that provides corrosion protection but also makes an insulating layer. That's why electrical connections to aluminum have lock washers that bite through the top layer to contact the pure metal below.
So, do what the leading manufacturers do, make it cheap and let it rot and then make a new one later. NOT.
Or else use all copper/brass/bronze and solder/braze the connections. This design uses all copper pipe plus plastic pipe as a spacer.
As to gain, note that 0 dB is set at the plastic dipole antenna as a standard reference so a vertical half wave could give us some advantage over that.
Is it worth the hassle? Probably not.
Aluminum and iron have the same issue but aluminum rapidly develops a thin layer of ceramic Al2O3 that provides corrosion protection but also makes an insulating layer. That's why electrical connections to aluminum have lock washers that bite through the top layer to contact the pure metal below.
So, do what the leading manufacturers do, make it cheap and let it rot and then make a new one later. NOT.
Or else use all copper/brass/bronze and solder/braze the connections. This design uses all copper pipe plus plastic pipe as a spacer.
As to gain, note that 0 dB is set at the plastic dipole antenna as a standard reference so a vertical half wave could give us some advantage over that.
Charivari
Fighting Naïve Realism
Transmitting or receiving, an antenna is doing the same thing, as far as it's concerned and will work just as well one way as the other. Think of an electric motor; you input a voltage, which is then turned into electromagnetic energy which is in turn converted into mechanical motion. Ie, put on the juice, the motor turns. However, if you manually turn the motor, that mechanical motion is being converted into electromagnetic energy, which is in turn converted into a voltage. Antennas do just about that, though one step simpler. They convert a voltage into an electromagnetic energy when transmitting and do the inverse when receiving, they convert an electromagnetic energy into a voltage. As far as the antenna is concerned, it's doing the exact same thing regardless, only the power levels are different.Whitehall said:
Now, theory aside, in practice, transmitting antennas, as you put it, tend to be superior to those designs marketed only for receiving. The reason is simple, receiving antenna makers can skimp on materials because the antenna doesn't need to be able to handle the power. More importantly, with receiving, antenna manufacturers don't really pay as much attention to appropriate matching (SWR) because if the antenna is mismatched at the tuned frequency, all you'll have is a loss of received signal. Do that with a transmitting antenna and you can destroy several thousand dollars worth of radio. So, you'll end up with an antenna that lasts much longer and will receive better if you go for one designed to handle both rather than a cheap receive only. The designs simply work on the same principle.
And yes, you figured it out, an antenna with a certain dispersion pattern while transmitting will have the same while receiving.
- JP
Charivari
Fighting Naïve Realism
I've built over a couple dozen J-poles over the years from the equations found on the first link and if you do it right, you don't have to worry about such things. I build mine out of copper water pipe using a copper T junction, a 90° angle, and two end caps for looks and to keep water out of the inside. I solder it up as though I were plumbing a house and all is good. To connect the coax, I'll bare the end, separate the shield from the center conductor and twist the braid together. Then using two hose clamps, I clamp down the coax ends to their respective portions of the pipe -- typically the center conductor to the longer portion -- and move the connections up and down until I either receive the lowest SWR measurement using a watt meter set to reflected power and a source transmitter set to the center frequency I tuned for or for receiving only, until a station I tuned into (at the center tuned band of the antenna) sounds the strongest. I then mark this location with a Sharpie, remove the clamps, sand the spot, add flux, and using the torch, solder the coax right to those spots. (The connection points should be directly across from each other on each leg.) Seal it up with some liquid tape to keep the water out. Then paint the whole thing with zinc chromate primer and finish it off with a couple coats of black Rustoleum spray paint.Morden2004 said:
There's no dissimilar metals to worry about here and examples of this construction have lasted 15+ years on fishing boats in some of the most dangerous waters of the world -- the Graveyard of the Pacific, the Columbia River bar.
For mounting to a metal mast, if you're worried about this, in the past, I just obtained a chunk of rubber hose, put it over the bottom portion of the J-pole (you connect a support to the bottom of the T-junction) and pipe clamp it to a mast. The hose separates the metals. I've also just soldered on a copper threaded end and screwed it into a galvanized deck flange for mounting and while the galvanic action did corrode the copper, 10 years later, it's still firm -- though God help you if you want to unscrew it.
Just saying, copper is the better material electrically and for ease of construction and if you do it right, it'll last longer than an Al constructed variant.
- JP
Charivari
Fighting Naïve Realism
It's an end fed 1/2 wave, so it actually offers better than unity gain and performs even better than a dipole, for reasons beyond me, I'm just speaking from experience here. The J section is a 1/4 wave matching section for the 1/2 element. For most antenna designs, you usually have to use a 1/4 section of ladder line or the line to serve this purpose and to act a bit as a balun, but the J-pole incorporates this right into the design and actually does it better. That's something I love about this antenna, it's so simple, yet it does everything right.bea1188 said:
- JP
This is something I actually built once! Worked very well, except I got paranoid about lightening strikes and took it down. It can also be built from wire, stuck onto duct tape or wide masking tape and then taped to an exterior wall or window (or a couple of yard-sticks to be movable). I think someone was once marketing a tall indoor antenna of this design - it was a 2 or 3 inch diameter 6 foot tall black tube with a nice round wooden base.
Works well in distant/rural areas with little reflections. Not so great if you're surrounded by lots of tall metal clad buildings and power lines etc.
Works well in distant/rural areas with little reflections. Not so great if you're surrounded by lots of tall metal clad buildings and power lines etc.
Morden2004
Relaxin'
packrat said:
My location may offer up the best topography for this antenna; I'm about 25 meters above mean sea level facing due north across the ocean. To the north west is Maine (about 70 Km) and north is New Brunswick (about 10 Km) and at my back is a small mountain range. Currently, with a simple dipole I receive FM stations out to about 90 Km well. I think this 'J' antenna will work well here.
Paul
Charivari
Fighting Naïve Realism
Yeah, it should work out great there. That's fairly similar to several of the locations where some of my J-poles ended up going to. They've held up quite well, though the antenna bought by the Port of Ilwaco's main office ended up having an anenometer mounted on the vertical element as well -- still worked great. The variants I built for myself were mounted up on the eeves of my parents' house, which was similarly located, up about 20 meters above sea level, but facing South out over the mouth of the Columbia River and the Pacific Ocean. Just be sure you build yours out of some pipe a bit stouter (~1") to handle the storms that you surely get there on the shore.
- JP
- JP
drspiff
Fisher "T" Aficionado
After readiing this thread, I went out and built a 3 element Yagi and a J pole to bring in the local PBS UHF television station. The results were pretty amazing to me. The Yagi performed very well, in fact giving the best picture we have ever had from this station. BUT... The J pole was almost as good. So rig up a J pole, you may be surprised as I was by the results.
Morden2004
Relaxin'
I built it!
I built myself a 'J' pole today. You can see some of the results in another thread here in [Tuners].
JP, thanks for the encouragement and the info. I had great results.
Paul
Charivari said:
I built myself a 'J' pole today. You can see some of the results in another thread here in [Tuners].
JP, thanks for the encouragement and the info. I had great results.
Paul
So I have questions on J-pole design.
I can use 300 ohm or 75 ohm line to the receiver - which is better or do I need a transformer/balun in the circuit to match?
How does one figure the separation between the half wave and the quarter wave poles? Is it an impedance thing?
One should be able to operate this ungrounded.
I've found one lengthy theoritical article on j-pole design but it was WAY over my head (and probably not clearly written in the first place.)
I ask these questions because I'd like to make a series of indoor J-poles using aluminum tape and output via 300 ohm twin lead.
I can use 300 ohm or 75 ohm line to the receiver - which is better or do I need a transformer/balun in the circuit to match?
How does one figure the separation between the half wave and the quarter wave poles? Is it an impedance thing?
One should be able to operate this ungrounded.
I've found one lengthy theoritical article on j-pole design but it was WAY over my head (and probably not clearly written in the first place.)
I ask these questions because I'd like to make a series of indoor J-poles using aluminum tape and output via 300 ohm twin lead.
Morden2004
Relaxin'
Whitehall said:
You can use either, but the design dimensions given were for a 75ohm feed. The 'B' dimension is, I believe, the one that will change the impediance but I don't have a reference manual for this design. Perhaps some one will help you out.
See aboveWhitehall said:
Indoors, yes. But grounding is a safety issue.Whitehall said:
Whitehall said:
Now that is a novel idea! And no reason why it won't work other than the connections between the segments. How did you propose to join them? Otherwise, a great idea and since the 'J' Pole is omni-directional it doesn't matter much how you orient it - I think.
However, rethinking this a bit there will be one very narrow face so I wonder how that will affect it's polar pattern. Interesting.
Paul
Mark W.
Nut house of the Universe
Paul he could just wrap the tape around a 90 degree corner like on a 3/4" square molding or even a door jam or cabinet corner to achieve an equal side.
It is a very interesting Idea. I would think that with the minimal time and materials in suck a project experimentaion would be the best way to figure out if it would work.
Personally after going to Home Depot and checking out the Copper pipe (which is better for antennas RED or BLUE grade LOL) I'm planning on using 3/4" water pipe with all soldered joints. A 10 gauge grounding wire going to either an unused Gas line (non longer hooked to the street) or to a dedicated gounding rod. I'll most likely tune it to the center of the dial And mount it as high as possible attached to the facia boards near one of the peaks in my roof. Should put the mounting point approximately 20-22" feet off the ground and virtually within line of sight from the two stations I currently listen to the most. One to the North the other to the South South West Both about 45-50 miles away as the crow flies.
So I should end up with about as good of recieption as I can get.
It is a very interesting Idea. I would think that with the minimal time and materials in suck a project experimentaion would be the best way to figure out if it would work.
Personally after going to Home Depot and checking out the Copper pipe (which is better for antennas RED or BLUE grade LOL) I'm planning on using 3/4" water pipe with all soldered joints. A 10 gauge grounding wire going to either an unused Gas line (non longer hooked to the street) or to a dedicated gounding rod. I'll most likely tune it to the center of the dial And mount it as high as possible attached to the facia boards near one of the peaks in my roof. Should put the mounting point approximately 20-22" feet off the ground and virtually within line of sight from the two stations I currently listen to the most. One to the North the other to the South South West Both about 45-50 miles away as the crow flies.
So I should end up with about as good of recieption as I can get.
Morden2004
Relaxin'
Mark W. said:
I used 1/2" copper for the antenna and 3/4" copper for the base stub. It is very sturdy.
Interestingly, I have been playing around with moving the connections up & down the 1/4 wave section and discovered that it's not overly critical. However, each time I moved them I tested the signal strength with the wires
left and
right and that makes a huge difference - but not always polarized the same way! So, experimentation is needed.
As for how to adjust the leads and see the signal strength meter at the same time from up on the roof? Well I didn't! I did it in the house and once you have the correct match, then you move it outdoors. You may have to use a near-by stair-well for the antenna (like I did) because the antenna height without the stub is a hair over 8 feet.
Paul
As for how to adjust the leads and see the signal strength meter at the same time from up on the roof? Well I didn't! I did it in the house and once you have the correct match, then you move it outdoors. You may have to use a near-by stair-well for the antenna (like I did) because the antenna height without the stub is a hair over 8 feet.
Paul
