Rich
Full Member
RF Systems Engr (retired)
Posts: 112
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Post by Rich on Jul 5, 2020 10:29:13 GMT
I first posted the following text on a "ham" website, but the essentials of it also apply to the short vertical antennas used for Part 15 AM systems, and may interest some readers here, as well. - Rich
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For perspective — the radiation resistance of a short, vertical monopole is a function of its physical height in wavelengths, and the distribution of r-f current along its conducting surface. This is true no matter what configuration of buried or elevated radials the vertical monopole is driven against.
Radiation resistance is the only useful resistance present in the the Real term of the feedpoint impedance of that antenna system, as all other resistances at the feedpoint do not produce the radiation of e-m waves into space. Instead, those other resistances (loading coil loss, radial system loss, etc) dissipate part of the available r-f energy as heat.
Below are the intrinsic parameters for a thin-wire, unloaded, base-fed, 14' vertical monopole driven against a perfect ground plane (calculated by NEC4.2).
20 meters (Hgt = 73°): Radiation resistance = 21.8 Ω, Feedpoint Reactance = -j161 Ω 40 meters (Hgt = 37°): Radiation resistance = 4.65 Ω, Feedpoint Reactance = -j764 Ω
A "loading coil" at the base feedpoint could offset the -jX present there to produce a resonant antenna system. But that will not change either its radiating height, or its radiation resistance in those operating bands.
If the loading coil is moved to the center of that 14' radiator, then the current distribution between the base and the coil input is nearly linear, and that raises its radiation resistance. It also raises its capacitive reactance at the base feedpoint, so the inductance needed for resonance increases, along with its ESR. The increased coil loss reduces system radiation efficiency, and the added inductance reduces the SWR bandwidth at the feedpoint. So center-loading has its downsides, too.
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Post by rob199 on Jan 28, 2022 23:03:19 GMT
In the Netherlands we don't have antenna restrictions. We got two license, 1 Watt PEP and 100 Watt PEP. A couple of years ago we build an antenna following the example as used in the UK. Height 10 meter, dynamic loading, witch means a coil and top hat in top. We used a 17 cm dia coil. Pictures are om extram.nl (scroll down)
Rob V.
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Post by Admin on Jan 31, 2022 22:02:14 GMT
Hi Rob, curious to know if your licensing is free or are there application fees?
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Post by rob199 on Feb 8, 2022 11:01:51 GMT
No it's not for free, there are aplication fees For the license: 164 Euro (valid 5 years) For surveillance and enforcement: 1 Watt PEP - 175 Euro, 100 Watt - 462 Euro (yearly) Note: You can only get a license with approval of the media commissioner: 200 Euro The problem is that they see it as "commercial". But nowadays (we have no longer high power stations in the medium wave) it is impossible to run a commercial radio station on medium wave. It's pure hobby broadcasting. But I am pleasantly surprised about the use of these licenses. radio-tv-nederland.nl/am/am.htmlRob Veld
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Post by mark on Feb 9, 2022 1:17:17 GMT
Converting euros to Canadian dollars adding all those fees up it's quite expensive for a hobby station, well, the total cost if you stay at 1 watt is not to bad yearly. One question....Is the AM band all for hobby broadcasting now? No commercial stations allowed?
Also how hard is it to get the license from the commissioner? If someone wants a license can they get it?, pay the fees and your all set to go? 100watts on AM can cover a whole small city! Maybe more at night. Not bad for a hobby station!
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Post by rob199 on Feb 10, 2022 14:00:56 GMT
Mark, just one thing! It is PEP power. So 0,250 W and 25 W with 100% modulation. Another difference between the USA and the Netherlands is that there are no antenna/ground restrictions. Commercial broadcasting is allowed with those licenses, but you have to pay more fees. (such as the media commissioner) For as far as I know, no one use these licenses for commercial broadcasting. We are a small country and medium wave is not popular anymore. BTW there are some SW licences to. radio-tv-nederland.nl/am/kg.html I agree with you that de 250 mW is expensive, to expensive for hobby broadcasting. A HAM, (wich I am) pays 35 Euro/year Rob Veld
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Post by Admin on Feb 10, 2022 19:33:32 GMT
Interesting that you also have shortwave licensing available and at lower power.
Seems that in the US, they want at least 50,000 watts and not for domestic broadcasting.
Per the FCC "It should be noted that an international broadcasting station is intended for broadcasting to a foreign country and is not intended for broadcasting solely to the United States. The minimum transmitter output power required is 50 kilowatts (kW) and a directional antenna is required with a minimum gain of 10 decibels (dB). Most existing stations have a transmitter power greater than 50 kW and an antenna gain greater than 10 db, in an attempt to overcome the increasing congestion and interference in the limited frequency spectrum allocated to this service. As a result, applicants should also submit with their application a propagation analysis, based on the proposed transmitter output power and antenna gain, showing that an acceptable signal strength will reach the intended target area(s)."
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Post by mark on Feb 10, 2022 21:37:45 GMT
@ Rob199... AM is not popular here either. It's considered a dying band. It should be for hobbyists. We can make it great again so to speak. Because of FM being much more restrictive and no space on the band to go as it's overcrowded in or near all major cities AM is the preferred choice. Fortunately no plans to mandate digital and scrap the band so it's good for us. We have 100mW at the input to the final amp but no rule on the power out at the antenna. We can get up to a mile/1.6km, sometimes more, with the 2 top transmitters here. Right conditions and location.
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Post by rob199 on Feb 12, 2022 16:26:04 GMT
@ MRAM Even with little power your range can be outside of the USA. Of course you need a good antenna.
@ Mark I have a small transmitter from SSTRAN AM3000. I experimented with it a long time ago. (That was the beginning for me to lobby for LPAM in the Netherlands) I was surprised about the reach of this cute thing with a ten feet antenna, base loaded. Note: The antenna was on a 20 feet mast and grounded with only two leads. Reach was about 700 m with a car radio with fairly comfortable reception. As I wrote, surprising cause I live in a town near Amsterdam called Purmerend with lots of man made noise.
Rob Veld
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Post by Admin on Feb 13, 2022 22:47:49 GMT
Rob wrote "Even with little power your range can be outside of the USA."
Yes, as a Ham radio operator I have made contacts across the globe with less than 100 watts.
Currently, I enjoy QRP (low power) CW on the 40 meter band. Using a "Pixie" CW .5 watt transceiver I have made contacts at 600 to 700 miles using a 60 foot horizontal wire antenna only 25 feet up.
My best QRP was with my 13.560 mHz beacon CW transmitter. It's total output is about 5 milliwatts to a ground mounted vertical. From northern Ohio the signal was copied in South Carolina. With the right conditions it can be done. Of course that little transmitter had been operating almost a year before that report was logged.
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