Slightly off topic - but YvesRoy, where are you geographically located?
Earlier posts in this thread and elsewhere mention installing a 3-meter Part 15 AM antenna 20 feet or so above ground. That has implications for the "coverage" it provides, so I put together a short paper about it.
SCWIS posted a link to the paper on the Part15.us Home page (thanks), but maybe some don't look there regularly. Here is the link...
http://filebay1.home.comcast.net/Elevated_Part_15_AM_Antennas.pdf
Rich
///
Slightly off topic - but YvesRoy, where are you geographically located?
I'm located near from Quebec City in Canada. The canadian law for the part 15 applies the same as the United States. I speak french and a little bit in english.
Regards!
Yves Roy
I've actually never seen the Canadian version of our Part 15.. can you cut and paste it here for us? Or provide a link? I am very curious to see if the wording is identical or if there are some "loopholes" in the the Canadian version.. if so.. teach me french 🙂
I've actually never seen the Canadian version of our Part 15.. can you cut and paste it here for us? Or provide a link? I am very curious to see if the wording is identical or if there are some "loopholes" in the the Canadian version.. if so.. teach me french 🙂
Industry Canada provides a PDF document at the following link: http://strategis.ic.gc.ca/epic/internet/insmt-gst.nsf/vwapj/rss210.pdf/$FILE/rss210.pdf
In this PDF document which the title is [color=red:0f33ff200d]Low Power Licence-Exempt Radiocommunication Devices[/color:0f33ff200d], go to the page 19 and you will see some things to comply with limits. It's the same limit as the Part 15 rules in the US.
Regards
Yves Roy
Rich,
I want to take this opportunity to thank you for your excellent analysis of the elevated Part 15 antenna. To my knowledge, this is the first time a technically accurate analysis of elevated part 15 antennas has been posted on the web.
Everyone seems to "know" that elevating the antenna improves performance, but the reason has been elusive. The instinctive answer is that elevating the antenna gets it above obstacles. This may be true to some extent, but your results show the radiation from the long ground is more of a factor.
There has been persistent discussions here in the past of the benefits of an elevated ground plane to enhance the performance of an elevated antenna. What do you think of that concept? My gut feeling says the benefit of radiation from a long ground lead is better.
Here's a thought. I think many elevated antennas consist of a long mast pipe or ground lead right up against the side of a building with the antenna protruding above the roof line. It seems the benefit of the long ground lead radiation may be negated by the close proximity to the building. Negative factors would be conductive siding and close proximity to house wiring.
Your analysis is valid for an elevated antenna well clear of obstacles, but not so much for a "typical" elevated installation.
My take is that the best elevated antenna would be up on a 20+ ft pole or tower that is out in the open and well clear of any obstructions.
Your thoughts?
Rich, I want to take this opportunity to thank you for your excellent analysis of the elevated Part 15 antenna.
And my thanks to you, scwis, mlr and others who have commented favorably on my posts. One prominent manufacturer of Part 15 AM txs has told me I'm all wrong, though.
There has been persistent discussions here in the past of the benefits of an elevated ground plane to enhance the performance of an elevated antenna. What do you think of that concept?
Sidestepping for the moment the legal issues about adding radiating lengths of any kind to the Part 15 AM antenna configuration defined and permitted by the FCC, I used NEC-2 to model a ground plane antenna having a 3m vertical radiator and four 2m radials, all elevated 6m above a perfectly conducting Earth. If this antenna configuration is driven by a battery-powered tx connected through a loading coil of 10 ohm resistance to the base of the vertical section (that is, there are no wires between the tx and the ground), then the peak h-plane gain of this configuration is -14.3 dBi. This value is about 10.5 dB lower than the 3m vertical at 6m elevation with a 6m "ground lead" leading to a 10 ohm ground -- as shown in my earlier PDF paper at
http://filebay1.home.comcast.net/Elevated_Part_15_AM_Antennas.pdf
.
Of course in a real installation there will be wires carrying DC power and program audio from the ground to the tx, and they will conduct RF current which will add to the radiation of the 3m part of the antenna system. Assuming that these wires are 6m long and terminate directly under the 3m radiator, and that they provide a connection to Earth ground having an impedance of 100 +j200 ohms (a total guess), the gain of the antenna system becomes
-11.3 dBi, or about 3 dB better than without the wires.
So the pattern gain of both of these ground plane configurations is well below that of a 3m vertical+6m "ground lead" radiator connected to a 10 ohm Earth ground.
I think many elevated antennas consist of a long mast pipe or ground lead right up against the side of a building with the antenna protruding above the roof line. It seems the benefit of the long ground lead radiation may be negated by the close proximity to the building. Negative factors would be conductive siding and close proximity to house wiring. Your analysis is valid for an elevated antenna well clear of obstacles, but not so much for a "typical" elevated installation.
The "ground lead" will still radiate, but the fields will be distorted and the net radiation pattern of the antenna plus house wiring etc will be somewhat directional. The nearby wires also can change the feedpoint impedance of the antenna system seen by the tx, and possibly reduce radiation efficiency.
BTW, I have no objection to direct e-mail if there are any questions: [email protected]
//
Hello,
Does it exist a reflectomer for AM band to measuring the VSWR with my AM25? I would like to buy this one. Maybe Rich or someone else knows that this one exist.
Thanks to letting me know.
Yves Roy
Most SWR meters won't react to much less than a watt. I just checked, and a Bird 43's lowest power slug for 500 - 1700 khz is a 1000 watts. The Bird is about the best one can buy. (There may be others as good; I am most familiar with the Bird. A ham radio bridge that covers 160 meters might work in the extended band, but I wouldn't trust it's accuracy.
http://www.bird-electronic.com/products/product.aspx?id=81
Does it exist a reflectomer for AM band to measuring the VSWR with my AM25?
This would be a real design challenge, and expensive for Part 15 AM power levels. Probably the most practical approach to optimise a Part 15 AM antenna system is to use a "ham" type communications receiver with a signal strength meter built into it. Connect it to a very short receiving antenna (or maybe none), tune the receiver to the Part 15 tx (no audio), and move it all far enough from the tx antenna so that the receiver signal strength meter reads in the lower 1/2 of its scale.
Now adjust the tap on the tx antenna loading coil, and any trimmer capacitor in the tx available for antenna matching for some combination of settings that shows the maximum signal on the receiver meter. Relocate the receiver further away, or shorten/remove its antenna as needed so that the signal peaks remain in the lower part of the scale as you tune the tx antenna system.
After each adjustment at the tx, check/re-adjust it for 100 milliwatts of input power per its instruction book. All measurements should be taken after the operator has moved at least 6 feet away from the tx antenna and any "ground lead."
The Part 15 tx book may have some other approach in doing this, which should be used, if so.
//
I have a simple relative field strength meter that someone built years ago and I inherited. I use it to tune antennas. It is a simple box with a telescoping antenna, a germanium diode, a small rf choke, a capacitor and a 25 microamp meter movement. Within about 6-8 ft of the antenna and with the FS meter antenna collapsed enough to bring the reading on-scale, it works very well. This type of FS meter was popular long ago, but cheap microamp meters are hard to find now.
MFJ Enterprises sells two inexpensive relative FS meters. I don't know how well they work, but they seem to be the only source that I have been able to find.
http://www.mfjenterprises.com/products.php?prodid=MFJ-802
http://www.mfjenterprises.com/products.php?prodid=MFJ-801
Rich,
Thanks for the "elevated ground plane" analysis. This idea has come up often in the past. I think you have laid the idea to rest (at least for an elevated antenna out in the open and well clear of obstacles).
Also, thanks for your input on the "mast against the side of the house" installation. Obviously, the best elevated antenna would be one that takes full advantage of the ground lead radiation. This implies a freestanding mast out in the open.
Here is another problem to chew on. Which of the following two installations would perform better:
1. A part 15 antenna essentially at ground level (maybe up on a short pipe of say 2 meters max.
2. An elevated part 15 antenna on a grounded pipe 7 meters long, but immediately adjacent to a fairly conductive building with aluminum siding or stucco on wire mesh.
I realize that the effect of the building siding or adjacent electric wires in the building is a total unknown for modeling. You may be able to use the worst-case situation where the building is a perfectly conductive ground surface.
What I am really asking is whether the part 15 antenna elevated to 7 meters with the grounded surface adjacent to the mast will perform better or worse than a ground level antenna out in the open. This is a practical consideration for many part 15 broadcasters. My gut feeling is the elevated antenna is still better because the antenna will radiate its signal above other adjacent building roof lines. But it is also possible that a large portion of the power will be shunted to ground due to the adjacency of the mast to the grounded conductive surface of the building.
Which of the following two installations would perform better:
1. A part 15 antenna essentially at ground level (maybe up on a short pipe of say 2 meters max.
2. An elevated part 15 antenna on a grounded pipe 7 meters long, but immediately adjacent to a fairly conductive building with aluminum siding or stucco on wire mesh.
It is a "given" that adding any conductor between the chassis of a Part 15 tx located at the base of a 3-m vertical radiator and Earth ground will increase the radiating length of the antenna, and improve its efficiency (other things equal). The longer the added conductor, the greater the effect -- at least until the total length exceeds 5/8 wavelength.
I realize that the effect of the building siding or adjacent electric wires in the building is a total unknown for modeling. You may be able to use the worst-case situation where the building is a perfectly conductive ground surface. What I am really asking is whether the part 15 antenna elevated to 7 meters with the grounded surface adjacent to the mast will perform better or worse than a ground level antenna out in the open.
I added 5 vertical wires each 6mm OD, 6m high, and spaced 3m apart in a line passing 0.2m from the 6m elevated Part 15 antenna+its ground wire, and each having a 10 ohm path to Earth ground (which resistance is probably lower than reality). The middle wire was 0.2m from the ground lead to the Part 15 tx. At 1.7 MHz this configuration of wires represents a solid rectangular, conducting surface, such as aluminum siding on a house. This surface shields the antenna from the other three sides of the house.
Adding the 5 wires reduced the gain of the open-space sited, 6m elevated antenna+ 6m ground wire by about 0.4dB, and introduced about 0.01dB of h-plane directionality to it.
It appears that the nearby conducting surface(s) are too small in terms of wavelength to couple much energy from the driven radiator system.
My gut feeling is the elevated antenna is still better because the antenna will radiate its signal above other adjacent building roof lines.
Not really, because a 3-m vertical radiator elevated to 6m, with a 6m "ground wire" from tx chassis to Earth radiates the majority of its field from the ground wire itself. The amount of current flowing over the length of a conductor is the effective source of radiation. If you look back at my PDF paper, the current on the upper 3-m of this system tapers to zero at its top. So the average value of current flowing along that relatively short length of radiator makes the 3m section not as good a contributor to the total radiation as the "ground wire" below it.
//
I have been following this thread with great interest and thought I would pass along what I am doing down here near Houston. I built an EH Antenna and am feeding it with an SSTran. My design is built on an 8" diameter irrigation pipe with copper sheeting for the upper and lower rings. This design, when placed at ~ 150ft, is supposed to function 80% as efficiently as a 1/4 wave antenna. I have yet to get it tuned properly (I have no antenna analyzer) with the loading and tuning coils, but have gotten very close several times when I had access to a friends AEA. Check out Google and/or Yahoo groups to see numerous examples, calculator tools, etc on the EH. I sure wish someone was close by that could help me with this thing, it is *very* heavy but should be totally legal for P15 applications, and you are not limited to only the upper end of the band, I built mine for 580khz... 🙂
Tom Johnson
http://cat-am.com
Please by all means keep us posted, I am still on a tight rope as to what I will be using for an antenna, although I am leaning towards the SSTRAN recomended build. Please supply us with photos and or specs if you can! Thanks!!!