Post by ogrevorbis on Oct 28, 2014 13:46:47 GMT -6
I was building an am antenna yesterday and laid down some radials with an antenna analyzer hooked up. Each time I added a radial, the resonant frequency dropped a bit, but the SWR continually got better. Today, I decided to add some more radials, but now the SWR increases each time I add another. Any ideas why and what I should do?
AFAIK you want as many radials as possible. I had the SWR at 1.11 yesterday. Today without changing anything it was 1.3 and when I began to add radials 1.5, 1.7, etc. It did rain last night and I'm thinking maybe the moisture has something to do with it?
I also added a ground rod and that did improve it a little.
Post by ogrevorbis on Oct 28, 2014 16:28:19 GMT -6
Just to elaborate on my setup:
I have a 10ft 1/2in vertical copper pipe with two 2ft 1/4in pipes in an X formation for the top hat. I have this mounted on a PVC pipe. Also mounted on the pipe is a 243uH inductor. I am feeding the antenna with 30ft RG58 coax. The antenna has 12 approx 20ft long radials, and when adding more radials the SWR goes up. I am not very familiar with building antennas. This is my preliminary setup. I plan to add height/more radials if I get this working.
It is possible that you are lowering the resonating frequency of the antenna by adding more radials in which case you would need to lower your inductor value (remove some windings) to accommodate. This is why it is best to use a tapped inductor or a roller style inductor so you can easily change its value. If anything this actually proves that your antenna is operating more efficiently because it is probably a sign of less ground resistance. Soil conductivity can have a small change in resonate frequency, or rather raise or lower the SWR slightly but usually it takes the difference between dry soil to outright flooded by rain to make a significant change in my experience. If anything a moister more conductive ground will give you a more efficient antenna.
Just curious what kind of meter you are using to measure SWR? Most SWR meters do not function properly at such low frequencies unless you have one made just for it. The expensive Bird meters often allow for it if you have the right plugin module. At 1.7MHz and below it is often better to measure antenna current using a current meter. The higher the current the more efficient the antenna is operating.
Post by ogrevorbis on Oct 29, 2014 17:47:38 GMT -6
It is possible that you are lowering the resonating frequency of the antenna by adding more radials in which case you would need to lower your inductor value (remove some windings) to accommodate. This is why it is best to use a tapped inductor or a roller style inductor so you can easily change its value.
Yes, that's exactly what I'm doing. Instead of using a roller inductor, I'm just removing a turn of wire at a time, taping, testing SWR, and repeating. This works OK as I can see the resonant frequency rising, however the SWR keeps getting worse at the resonant frequency. At 1500khz the SWR was basically 1. I want to use 1700khz and the best I could get was 2.5 SWR. Through some experimentation I found that the only way I could lower the SWR at that frequency was to add another small inductor (16uH) to the ground plane. I then got 1.2ish SWR. I drove around with that and it went like 1/2mile with 2W carrier. I found that if I remove that coil from ground and then use an antenna tuner, it goes a little farther. I really don't know what I'm doing. Should there even need to be an inductor on the ground plane? I'm thinking maybe something else is wrong and I'm just compensating by adding that.
I'm using a stranded wire inductor. I'm not using any ground rods (only plane). Does that make a difference?
For my SWR meter, I'm using a meter that I borrowed. It's a rig expert AA-600. I'd like to get this tuned up prefectly while I can because my normal meter is a DAIWA 1.8-150 SWR meter, and also an MFJ antenna tuner. With the rig expert it gives a very accurate SWR and also all these other readings that I'm not sure what they mean. Like R and X.
I'm surprised that I'm not getting more range with the AM. In the past I've messed with FM and got at least three times the range with the same power level. I know that antenna efficiency is very low at the low frequency, but I hear part 15 100mw people getting almost a mile, I should get more with 2W. My amp is capable of 15W (60W PEP) but I'm using it at low powers now just for testing.
I'd like some advice on the most important things to check/improve.
In my experience it is best to make the ground system as conductive, lowest resistance, as possible. This means long radials, as long as you can possibly lay down. You should put down radials anywhere you can. Having a nest of radials even if not straightened out like spokes around the antenna is still better than using many short ones that are neatly laid out. I would focus first on getting your ground system as effective as possible. 20ft radials is not of much help at all. Using a few really long radials up to 1/4 wavelength are far more effective than hundreds of 20ft radials. In other words longer fewer wires > more shorter wires.
You really should build or buy an old used RF ammeter "current" meter. They are very simple devices and only consist of a torroid with a few windings around it slipped over a very short piece of unshielded coax out of your transmitter. The coil then goes to a diode, and to a simple RF blocking capacitor and meter to measure current. Using one along with your SWR meter will show you far more than you could ever see with the SWR meter alone. The more current that is measured means the more effective the antenna is. Comparing that alongside SWR and measuring to get current the highest, and SWR the lowest will yield you the most effective antenna you can possibly setup. This is actually what licensed AM broadcasters do to setup their antenna systems.
Other than that I would really pay attention to how you are wiring up your loading coil. The trick with medium wave coils is to adjust both impedance seen by the transmitter and a proper high impedence/resonance seen by the main antenna radiating element. Even if you do not have a current meter you can substitute by using a 13.8v car break light. Connect it in series between the inductor input and the coax output coming from your transmitter. Tune the inductor until you get maximum brightness from the bulb, then short out the bulb or remove from circuit and reconnect as normal. Brightless = more current = more efficient antenna, or rather suggests that the antenna is resonating as best as possible to your frequency of use.
Here is the same simple setup I use with good success that I drew up quickly...
I use an actual current meter, but in a pinch you can use a bulb for a birds eye view of what is going on. Obviously no where near as accurate as a meter. For anything over 15 or so watts you may want to use a few bulbs in parallel so they don't blow out. Also as always test using low power so that you don't blow up your transmitters final. I am used to mosfet outputs that can withstand some incredibly high SWR before failure, but your rig may not be so lucky.
Here is a very simple current meter you can build.. You simply run your transmitters output coax right through the hole of the torroid, or a small piece of bare wire that carries your RF so you can sample it.
Remember SWR is a relative measurement and may not always reflect the actual reading seen at the antenna feed point when measuring from the shack location. This is what confuses people most when running long coax and measuring inside their radio shack because standing waves can actually show a false reading on the meter making you think the SWR is good when it really is not, or vise-verse. Always best to measure SWR at the antenna feed point if possible I think.
RF current measurement actually measures both forward and reflected current added together if I remember correctly, so it is showing you SWR at the same time as reflected current, however it is more useful because it is indeed measuring current rather than voltage like SWR meters alone do. Current is an indication of antenna resonance or some shit.
Either type of meter will only show you one side of the fence, but put them both into use and you can see what is really going on. Someone please correct me if I am wrong but this has always been my understanding and has worked for me when following these methods.
Oh one more thing, you may want to try different types of antenna matching inductor circuits. I know the classic way is to use a single inductor and ground the far side, tap part way up to match the transmitter impedence, and tap much farther up to match the high impedance of the aerial, however I have never had much luck with this method for some bizarre reason.
I much prefer the single capacitor and two connection point inductor like shown in the first image above, I find it is far easier to tune up. With this method I just tune the capacitor for maximum measured RF current, then tune the inductor for lowest SWR seen at the transmitter. Usually both have to be done simultaneously to come to a final perfect value.
With the inductor not using a capacitor that uses three connections (ground, TX input, Aerial output) it takes a lot more work to find the perfect spot for tapping it unless you design it to be used as a variometer which goes beyond the scope of this discussion.
Post by ogrevorbis on Oct 30, 2014 19:30:01 GMT -6
That's a lot of info. Thanks. I'll try that light bulb trick and maybe the ammeter if that's not effective. I noticed that your wiring diagram is much different than what I have. In my setup I have the bottom of the inductor to the transmitter output and the top to the antenna. There is no ground hooked up to the inductor. I then have a second smaller inductor that connects the ground of the transmitter to the radials. Is this a problem? (I did it based on this part15 thing I read, but I don't intend to stick with that design.)
If I do it the way you showed in your diagram, can I get away without the capacitor as long as I make the inductor perfect? If not, could I use the MFJ tuner inside next to the transmitter rather than a capacitor outside?
I then have a second smaller inductor that connects the ground of the transmitter to the radials. Is this a problem? (I did it based on this part15 thing I read, but I don't intend to stick with that design.)
That doesn't sound right at all. Your ground is your ground, period. There should be no inductor between your ground/radials to your transmitters ground.
If I do it the way you showed in your diagram, can I get away without the capacitor as long as I make the inductor perfect? If not, could I use the MFJ tuner inside next to the transmitter rather than a capacitor outside?
You can design the tuner network in such a way to bypass the use of a capacitor and instead put another inductor (loading coil) in series with the aerial output. The roller inductor (impedance match coil) from aerial to ground as shown in the pic is absolutely necessary as it brings the antennas high impedance down to something the transmitter wants to see, usually 50ohms.
I didn't realize you were using a separate tuner. There should be absolutely no reason to use a tuner if you build the antenna tuning network properly, aka: the inductor/capacitor combo or other various methods found from around the web at the base of the antenna. Using a separate tuner inside can only cause more problems, especially since there is no need for it.
You can design the tuner network in such a way to bypass the use of a capacitor and instead put another inductor (loading coil) in series with the aerial output. The roller inductor (impedance match coil) from aerial to ground as shown in the pic is absolutely necessary as it brings the antennas high impedance down to something the transmitter wants to see, usually 50ohms.
So you're saying that I may need an inductor between antenna and ground as well as one on the antenna.
I want to be very clear with this one part.
So if I choose to use very accurate inductors (instead of a tuning capacitor), then I should have two? One between TX and antenna and one between TX and ground? If this is true, how do I calculate (or get a rough idea) the one between TX and ground?
In the AM antenna construction tutorials I've read there is always just one inductor between TX and antenna, but none on ground.
The inductor going to the antenna acts as a loading coil so that you can use a shortened antenna, which of course on AM mediumwave is necessary since almost no hobby broadcasters can setup a full sized antenna.
The inductor going from antenna to ground acts as an impedance match so your transmitter sees a proper 50ohm load, since shortened antennas by themselves have a very high impedance.
I have no idea how to calculate these values since so many things would effect the values needed. You should take a look at the LPAM Handbook which can be found around on the internet and go to the section about antennas. It goes into great detail on how to setup one.
I've altered my setup. Now I have: 32 30ish foot radials 20ft 3/4in pipe antenna mounted about 3ft above ground 12 4ft top hat spokes (10-12ish gauge wire)
I get maybe an extra half mile range with this over the previous setup I mentioned in my second post.
I didn't realize you were using a separate tuner. There should be absolutely no reason to use a tuner if you build the antenna tuning network properly, aka: the inductor/capacitor combo or other various methods found from around the web at the base of the antenna. Using a separate tuner inside can only cause more problems, especially since there is no need for it.
So will using a tuner inside without any matching network outside work well?
I tested the antenna with the light bulb trick you mentioned. The light bulb got the brightest without any matching coil outside or inside, but the SWR of course went up to 2.5. With the matching coil outside the light bulb was about half as bright. I found I could get the bulb almost fully bright if I used the tuner inside. Once I got the SWR down with the tuner I unhooked the light bulb, but then the SWR went back up and I had to retune it. I had to test the light bulb between the loading coil and the antenna instead of between coax and loading coil because I have it soldered and heat shrunk and I didn't want to mess with it. Is it necessary that I hook it up before the loading coil?
I forgot to mention that the antenna is positioned about 20ft away from a building with a large metal roof about the same height as the antenna. I don't want the signal to go in the direction of the roof, so I figured it doesn't matter. Could this be a problem? I was thinking maybe I should try to ground the roof or attach the antenna to it and try to retune it.
It shouldn't matter if the bulb (or current meter) is connected between antenna wire/pole to tuning tank, or between tuning tank and coax from the transmitter. Usually it is more convenient to have it connected in the latter method. Just make sure to take measurements (brightness) are taken after the coax, not before. Also the tuning tank should be after the coax from the transmitter.
If you are still using the separate tuner inside you are not going to get very far before pulling hair out from frustration. As I said with the proper tuning tank, whether it be using inductor/inductor, or an LC (inductor capacitor) combo at the base of the antenna itself there will be absolutely no need for a separate tuner indoors at the transmitter. You are just not going to find a good match using an indoor tuner connected between the transmitter and coax running out as most of the power will be lost in the tuner itself. The thing is the antenna outside without any matching network will have an incredibly high impedance, and connecting that directly to 50ohm coax alone will have an incredible mismatch, something an indoor tuner can not properly take care of. This is why there is a coil at the base of the antenna to act as an impedance transformer to get it to 50ohms so that it matches your coax cable, along with the other coil that virtually lengthens your antenna itself, aka the loading coil.
Having the antenna so close to a metal structure will screw with your radiating pattern along with SWR. 20' is far enough that it shouldn't have a dramatic effect but it will definitely act as a reflector making your radiation pattern stronger in one direction. Keep in mind 2.5 for SWR isn't so bad as long as the antenna is radiating as well as possible. I would guess using even more grounding or even an actual ground rod at the base of the antenna could help lower that, but keep in mind SWR measurements down at those frequencies can be a little deceiving, at least from my personal experience.
One last thing, if you are getting a mile or two of broadcast range with only a 20' pole then that is actually not too terrible for 15 watts carrier. It is at least a start so you know you are doing something right so far. 20' antenna at such low frequencies radiates very little of your power. Figure with 15 watts only tens to hundreds of milliwatts will actually be radiated with that short of an antenna and even that is wishful thinking.
If you really want the distance and easier tuneup you may consider trying a different antenna like an inverted L style or even marconi T wire antenna. With the L configuration it shouldn't be hard to get 100' of wire in the air, at least 20 or so feet vertical then run the remaining horizontal to a tree with insulator or something similar. Remember the longer the antenna, the less inductance needed to tune it up and the more bandwidth the antenna will have making tuning far less sharp to get zeroed in.
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