| Author |
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Insider
| | Posted on Tuesday, November 19, 2002 - 1:33 pm: | 
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This isn't directly related to CB, but I'm sure the answer could be applied there as well.
Can an antenna be cut to half the lenght of it's 1/4 wave dimensions and still give more functional efficiency than a random piece of wire? Will the feedpoint impedance be somewhere near 50 ohms?
I have a 19 inch ground plane scanner antenna that I want to use with a part 15 FM transmitter kit. 1/4 wave at the low end of the band is somwhere around 30 inches or so. I figure if I can cut my antenna to 15 inches or so, it will work more efficietly than at 19 inches while being nice and compact. Hey, when using flea power, you need all the advantages you can get.
Thanks for your help |
de
| | Posted on Tuesday, November 19, 2002 - 2:05 pm: |
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An antenna less than a quarter wave is possible but you will have to feed it with some kind of inductor/loading coil. It will probably not have any gain. The shorter the physical length below a quarter wave the more base loading you need and the less efficient of a radiator it becomes (db gain wise).
You did not say the frequency you wanted to use the antenna on. I strongly suggest that if at all possible use a non loaded quarter wave length anenna if at all possible.
Item 2, Random wires....
Random wires are not all that efficient. They are sometimes necessary of convienent but not efficient. Random wire antennas almost always require tuners to make the radio see a happy 50 ohms. The impedance of the feedpoint depends on the frequency you are using it on. It can be less than 50, about 50 or over 50.
Most people use random wire antennas because they do not want to put up a dedicated mono band antenna farm or because they cannot put up a full length quarter wave (the 160 meter band for instance).
If you want to get some gain out of a wire antenna cosntruct a quad loop antenna and orientate it in a vertical fashion. The bidirectional lobes will provide you with some gain and depending on the frequency the loop antennas are not only easy to constrcut and errect but they are usually difficult to see at a distance. |
bruce
| | Posted on Tuesday, November 19, 2002 - 2:56 pm: |
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Hummm a 1/8 wave well the feed resistance would be very low and the lobe distorted but with a matching stub of some sort it should workjust for kicks you might try to top load it with several wires mounded at the very top and fanning out. It would be my opinion that it would work better if you dont cut it and try a top load.
bruce |
Insider
| | Posted on Tuesday, November 19, 2002 - 5:16 pm: |
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Thanks for the opinions. I was looking to see if I could avoide loading, but it looks like there's no getting around it short of making a simple dipole. As for gain, I'm not too worried about it. Actually, the less gain the antenna has, the more likely I'll be compliant to part 15.
Maybe I'll look into getting some more stainless steel rods and using clamps to make adjustable extensions for the main radiator and the ground plane radials.
If I did try the loading coil, how would I go about determining the number of turns needed to load the antenna short of trial and error? Also, would I need to add any loading to the ground plane radials?
I won't be able to tune it perfectly as the 10 or so mW output is insufficient to drive my VSWR meter, but I'd like to get it aproximately right.
I'd like to set the unit up for 88.9MHz. |
ryan
| | Posted on Tuesday, November 19, 2002 - 8:53 pm: |
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ahh yes, the good ol FM band. i would definetly try loading the antenna since it would match up better to your transmitter. you can spew a lot of harmonics with a bad match and could possibly have a spur in the aviation band. be careful, you dont want clearchannel's gestapo death squad to knock on your door.
what kind of transmitter you using? i got an NRG PLL pro 3 which can put out 5 watts. i dont use it much unless im warming a dummy load to my neighborhood. i have a 1/4~ GP ready to use in case of natural disaster or the coming apacolypse |
de
| | Posted on Tuesday, November 19, 2002 - 10:38 pm: |
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Re insider
I found the following on a 144 mhz mobile antenna loading coil. I would lengthen the coil a bit to add a few more turns.
The dimensions given for the form are as follows: a plexiglass form (although pvc would probably do well) having a 3/4 inch diameter, a length of 3 1/2 inches with 10 1/2 turns of no 10 or 12 bare copper wire.
If you want to see a picture/diagram you can look it up in the ARRL Antenna Book, 17th edition in the chapter entitled Mobile and Maritime Antennas.
DE |
Insider
| | Posted on Wednesday, November 20, 2002 - 7:33 pm: |
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Thanks DE, I'll look into it.
To Ryan:
I've looked into the NRG gear. Excellent products, especially when ordered pre-built (can't blame Steven for his product not working if your soldering is shotty).
As for harmonics and spurs, that's my biggest concern. Interfering with the aircraft band is dangerous and a sure way of getting shut down.
The transmitter is a Broadcast Warehouse model. with a variable power control (at minimum, the BW has the same output power as a Ramsey kit: 10 or 20mW). I tried using the dummy load, but even with some power running into it, reception in the same room as the dummy load was spotty, let alone the rest of my house. I can't fault the dummy load for that-it's just doing what it's supposed to.
I have no plans to pirate on the broadcast band for three main reasons:
1) Interference: Running power levels of around a watt or more may cause interference to my neighbors reception of adjacent stations.
2) No link: Running the transmitter from home doesn't sound like a smart idea.
3) The college station does a better job of playing a diverse mixture of music than I ever could. Anything other than music would stick out and the death squad would be there faster than you can say VSWR (viswar).
Not to mention I'd probably be banned from the industry for life. |
707
| | Posted on Wednesday, November 20, 2002 - 7:48 pm: |
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Insider- I'd go with the rigid dipole
ryan- "...clearchannel's gestapo death squad to knock on your door..." I resemble that remark ;-) |
ryan
| | Posted on Thursday, November 21, 2002 - 12:15 am: |
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707, please tell me, what do you mean by you resemble that remark?
im confused now |
ryan
| | Posted on Thursday, November 21, 2002 - 12:12 am: |
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insider, what kind of format do you carry on your flea powered station? if you need any death metal, hardcore, or some punk, i can send you copies of some stuff that ive aquired or recorded. |
bruce
| | Posted on Thursday, November 21, 2002 - 7:36 am: |
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clearchannel's gestapo death squad Is watching!!! Love it but your right there is a over inforcement of brodcast frequencys. The idea should be your are interfering with a licenced station .... unlikely when you cant light a flashlight bulb with your output. I been a big fan of micro stations for years the 1-10 watt stuff but i guess the big boys are afraid that your 1 watt station will take all the avertizers from them.....Hummm in that case i wonder who needs to change.
Bruce |
Tech833
| | Posted on Thursday, November 21, 2002 - 12:12 pm: |
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Remember that no matter what antenna you use, in order to comply with part 15 in the Fm broadcast band, you are limited to a signal strength not to exceed 250 uV/m measured at 3 meters.
From CFR ch. 47 Sec. 15.239 Operation in the band 88-108 MHz.:
"
(a) Emissions from the intentional radiator shall be confined within
a band 200 kHz wide centered on the operating frequency. The 200 kHz
band shall lie wholly within the frequency range of 88-108 MHz.
(b) The field strength of any emissions within the permitted 200 kHz
band shall not exceed 250 microvolts/meter at 3 meters. The emission
limit in this paragraph is based on measurement instrumentation
employing an average detector. The provisions in Sec. 15.35 for limiting
peak emissions apply."
So whether you use a gain antenna, random length, dummy load, or 1/8 wave radiator, you simply adjust your transmitter power accordingly.
FYI: Theoretically, running 20 milliwatts into a 1/4 wave radiator will put you almost 4 times over the legal limit. |
ryan
| | Posted on Thursday, November 21, 2002 - 7:53 pm: |
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i run 1 watt into a dummy load. its the lowest i can do and i have no field strength measurement capability so the fcc can kiss my *** on this one
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bruce
| | Posted on Thursday, November 21, 2002 - 9:25 pm: |
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Thats not nice Ryan.... HE HE HE |
Insider
| | Posted on Thursday, November 21, 2002 - 10:26 pm: |
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Oh great, I started something. Sorry Copper. Relax, this isn't supposed to turn into The People of the Copper Forum v. The FCC.
Tech833 was just pointing out something to consider. After all, the FM band is scrutinized more heavily than the Citizen's Band.
I was aware that I might be over the legal limit, but I didn't think it would be by that much, and at even four times the legal limit, the signal is still real anemic. The transmitter can't go down to 5uW, at least without some sort of external attenuator network.
I used the dummy load in the past, with a watt in, but stereo reception, even with a radio next to it was less than desirable. Mono doesn't sound too bad. I wanted to see if I could eek out some better reception, while remaining close to compliance.
The ironic thing is it would be so much easier to just blatently go illegal than to try and stay within the law.
Speaking of law, Bruce stating that there's too much enforcement and that he's a fan of some individuals that operate at illegal power levels? I don't know Ryan, maybe you better get that ground plane going, 'cause I think the apacolypse is on it's way (LOL). |
Tech833
| | Posted on Friday, November 22, 2002 - 1:16 pm: |
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I am involved with one of the college stations near me and they use a part 15 transmitter to cover the campus and over a mile away extremely well.
Since there is a 170 foot high water tower on campus that already houses some radio equipment with lots of copper pairs all the way to the equipment room near the top, it was easy to get audio from the studio to the part 15 transmitter on the tower.
With the part 15 transmitter at 170 feet AGL, you can hear the station over a mile away clearly. You can't pick it up inside a building very well, but automobile reception exceeds our expectations.
The transmitter is made by a company called LPB. It is synthesized and certified as part 15 compliant for the FM broadcast band. The transmitter and antenna are all mounted inside an outdoor rated enclosure which is mast mounted on top if the structure. It just looks like a small gray box. You purchase the transmitter/internal antenna/box as one solid unit complete, certified, and ready to go. We just connect power and audio and turn it on.
Even in micro power, elevation is everything. |
Insider
| | Posted on Friday, November 22, 2002 - 2:52 pm: |
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I hear that. A few miliwatts at 100 feet will probably do better 2 watts at 20 feet elevation. Circumstances won't allow me to put up any 100 foot towers anytime soon. If I could, forget the FM, the Antron would go there.
Any thoughts on using lossy coax, such as RG-58 as an attenuator? |
Tech833
| | Posted on Saturday, November 23, 2002 - 1:19 am: |
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My thought: Don't. |
ryan
| | Posted on Saturday, November 23, 2002 - 3:00 am: |
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hey insider, i found a site which deals mostly with pt.15 yardcasting and the sort
http://www.communityradiousa.com
check this setup out for AM flea power:
http://www.vcomp.co.uk/gizmo/amtx.htm
looks like a pretty nice setup.
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Insider
| | Posted on Saturday, November 23, 2002 - 12:12 pm: |
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Didn't think it was a good idea. I heard it mentioned somewhere and wanted confirmation.
I haven't turned the transmitter on in a few months. I'm waiting till I can get this right. |
ryan
| | Posted on Sunday, November 24, 2002 - 5:08 pm: |
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also try www.frn.net for antenna ideas.
and email me your address so i can send you some music to go over the air. diy_till_i_die@hotmail.com |
Tech833
| | Posted on Sunday, November 24, 2002 - 7:41 pm: |
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I'm often told that I do not explain myself the way I should. Please understand that this whole moderating and helping others is new to me, I normally do not explain myself and I'm used to it.
The reason you don't want a leaky coax attenuator is that unless you stretch that RG-58 along a road you are trying to cover, all you will do is cancel out the signal you lose through the shield, the same as a random wire antenna that is multiple wavelengths long does. To get the benefit of some radiation coming from leaky cable, the cable must be run along the corridor you are trying to cover (like along the inside of a tunnel) and remain very close to the antenna you are trying to couple signal into. As you move further from the leaky cable, the signal disappears from phase cancellation. The more power you run, the more cancellation you get. Law of diminishing returns.
A resistor network attenuator is much better. However, you still have to abide by the same part 15 radiated limits regardless whether the radiator is an antenna or leaky cable.
If you instead wish to broadcast illegally, that is another story, and that I cannot condone. |
Insider
| | Posted on Monday, November 25, 2002 - 1:39 am: |
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Very concise answer.
This is exactly why I brought my questions here. I was actually reluctant to ask the questions in the first place as I don't want this to turn into a thread about pirate radio.
I'm not interested in illegal broadcasting for the reasons mentioned in a previous post and others.
I just looked at that transmitter you mentioned. Sounds like that college station is doing well, as LPB states the transmitter has a typical coverage radius of 50 feet.
Sounds like I should play it safe and reduce the 20mW to a lower power level with a resistor network.
Thanks everyone for all the advice. |
bruce
| | Posted on Monday, November 25, 2002 - 10:09 pm: |
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gosh we cant have that 100 mw power house! The death of FM broadcasting sometimes the FCC outdoes its self on stupity i liked the old way 100 mw input to a 3 foot ant but heck i still build crystal sets. |
Insider
| | Posted on Monday, November 25, 2002 - 10:55 pm: |
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Part 15 AM is still that way Bruce. 100mW input but you're antenna system: coax, radiator, and ground can't exceed 10 feet in height. If you're on a college campus, you're allowed more field strength on the campus ground, but part 15 limits apply on the perimiter. |
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