hello everyone, I'm new to lpam/micro broadcasting. I've been interested in it for quite some time but finally bought a SSTran kit and now have it assembled. I have some experience with soldering and electronics(I can tell a resistor from a diode) but not a lot of knowledge about radio. So far I'm only using the supplied 10' wire and wanted to put off building a coil based antenna until I know for sure what I'm doing. I figured the best place I could put the sstran would be on a shelf in the closet of a spare bedroom and have the antenna go through the ceiling, attic, and stick out the top of the roof(my attic is very small). That way the transmitter would be protected from the extreme hot and cold weather here. Also, my neighbors probably wouldn't like the site of a huge antenna on the side or top of my house, but a few feet of copper pipe would go mostly un-noticed. Grounding would be the only issue to solve with this setup. The SStran manual says it can reach ground through the ac adaptor, but if thats not good enough there is the metal vent from the furnace that runs through the attic but with a wire running over to it would probably be more than the legal 3meters...
Also, I was wondering how does the coil/inductor work? If 1/4 wavelength of 1610 is 152 feet, then if I wound 152 feet of wire around a pvc pipe and attached the end of the wire to a 10 foot(or 3 meter) copper pipe would that get the signal further than just the supplied wire? In other words, does the length of wire on the coil make up for the length of the antenna? and regarding the sstran antenna instructions: How exact does the coil have to be made? If the wire is +/- one gague from the instructions or if the pvc pipe diameter is +/- 1/4" will it just not work? If I my potential setup of having the top of the antenna stick out of the roof would work then I guess I wouldn't have to weather proof the coil, thus saving time and money.
If anyone can shed some light on these questions I'd really apreciate it.
Your observations are good and your ideas well thought out. Unfortunately, radio has a few tricks up its sleeve that will frustrate your success without some help 🙂
In no particular order 🙂
Enjoy!
The materials referenced by SCWIS will be very useful to your inquiry.
I will only raise one question about your described installation, but since it is a question, I don't know the answer, so I am prompting other members to comment on this important aspect:
Is it not true that an antenna exposed to the outdoors is presents LIGHTNING BAIT to the atmosphere? Or, does an enclosed attic antenna also attract lightning.
That question leads to the need, perhaps, for a safety ground, separate from any concern about a counterpoise ground.
Aside from the safety question, being high AND out in the air will improve your out-reach.
thanks for the replies scwis and carl. those links have a lot of info and I'll be looking over those for the next few days.
As for the antenna attracting lightning, it might (I really dont know though) if it's sticking out of the roof, but I have trees taller than my house nearby, not to mention my neighbor has a flag pole about as tall as my house that as far as i know hasn't been struck by lightning. Inside, in the other end of my attic is an old tv antenna and assuming it was put in when the house was built, hasn't attracted lightning for 45 years. Perhaps the antenna could come out at the peak and just stick up one inch. I dont know if that would be less likely to be hit by lightning though.
Also up in my attic is a light but I don't know if the ground in the romex cable is hooked up or if it would be an effective ground. but it is close enough that I don't think it would violate the 3m rule of the total transmission line. another reason I would want to put the antenna in the attic rather than ground level is because of copper theft(or just theft in general). It's not a really big problem in my neighborhood but I'd rather not risk putting my transmitter and a copper pipe antenna in the middle of my back yard and have it get stolen.
The back of my house has a deck and I could safely put the transmitter under it and have the antenna stick up through the boards. Grounding would be a non issue, however my house would be blocking the antenna and there are more homes in front of my house than behind it. so I guess that brings up some questions. is antenna height or better grounding more important for better range? Or does it depend of the terrain, trees, overhead power lines, etc?
I wouldn't go to the trouble of poking a hole in my roof so the antenna could extend a few feet. The improvement from this would be minimal if at all. There are detrimental effects acting on indoor antennas which reduce their effectiveness. Proximity to conductors and building materials in the walls can detune the antenna and reduce the radiated signal. As an example, my home is stucco on the outside and the wire mesh behind the stucco forms a fairly effective Faraday shield which makes outdoor use of cordless phones and wi-fi marginal since the base units are indoors. Foil coated intermediate sheathing also can block signals though not so much at AM frequencies.
So, there is basis to the belief that an outdoor antenna works better but the greatest improvement is realized when it is installed above ground radials. For AM the height is not an advantage though it can appear so when reports of elevated antennas claim more range. This is due to the ground lead, mast, pole, or whatever establishes the connection to earth ground radiating the signal and not to the height of the antenna by itself.
The general principle of lightning protection is to keep it from entering a building. Most electrical codes require wires entering a building to be bonded to earth ground at the point of entry with the intent that lightning induced currents are shunted to ground instead of entering the building.
One theory of lightning strikes forming is based on micro discharges and coronas forming on metal objects outdoors and these can produce conductive paths for a strike to form. Presumably these would not form on indoor objects but I don't know this for certain.
These thoughts are to stimulate further reading on the subject.
Neil
Ya just never know where lightning will strike for sure.
I have a 55 foot tower on my garage and in over 30 years its not drawn fire. I had a 10 meter beam on my house for several years, no problem. And yet, lightning has stuck within 1/4 mile of my house.
I was standing 300 feet from where lightning stuck a tree on a residential street. Lots of other trees and power lines to atract it and it picked that particular tree.
There was a house surrounded by very large trees, lighting hit the house.
The only time I got nervous was when I witnessed static discharges inside my second floor bedroom. Freaky! We went downstairs 'till the storm was over.
My Fav read is the Cunningham book.
Well I finally had a day off work to mess around with the sstran. I bought a 3/16" x 4' steel rod to use as an antenna(I'm still thinking about using the top shelf of the closet and making a small hole for the antenna to go up into the attic, but not through the roof now). Here's the setup I'm using now: the antenna described above, a loading coil which I made following the formulas from the first link of the first reply(although I probably did it wrong, if anyone wants to check to see if they get 14.5 for the number of turns with a coil radius of 2" on 1610khz). The ground is a wire I ran through the floor into the garage and hooked to the metal pole that holds the I beam. I'm not sure if it makes much difference but I used the metal bottom of a pringles can as a capacitance hat on the top of the antenna. I also used a meter to adjust the trim cap to peak voltage across t1 t2.
The reason I think I may have made the coil wrong is because those forumlas were a little confusing. One was to figure the inductance in Henrys and the next one had the same variable listed as mico-Henrys. If I used micro henrys in that next forumula I'd have been making 14,000 turns instead of 14.5. Also for the radius of the coil I assumed inches because it didn't specify.
The signal is strong for about 400~500 ft and fades about 700ft. However the antenna and sstran are about 3 ft away from a computer in the back of the house so the signal is having to go through all the walls to get outside. so moving them further up and away might yield better results.
edit: just want to add that I used 2 different short wave radios to test for harmonics and I can pick them up on 3.22 and 4.83mhz but only pick them up strong within 10ft of the transmitter. on the higher sw band I couldn't pick any up but I can pick up some local fm stations(they sound a little distorted but clear enough to identify). is it normal to have harmonics on the first two frequencies above your intended frequency? If fm stations are allowed to spew harmonics onto the short wave band should I have nothing to worry about then? btw I used an online harmonics calc at http://www.electronicsteacher.com/electronics-calculator/electrical-harmonics-calculator.php
is it normal to have harmonics on the first two frequencies above your intended frequency?
Yes. Out of band emissions are allowed to be up to -20dBc (this is 20 dB below the carrier which is one one hundredth the carrier power. So if a typical transmitter output is 20 milliwatts the harmonic power will be less than .2 milliwatts. Actually, -20dBc is a rather high limit but since the part 15 power levels are low to begin with the assumption is the harmonic power will be low enough not to cause trouble. Usually for simple transmitters the odd harmonics are stronger than the even harmonics. Also keep in mind that the first harmonic of a frequency is that frequency (the first harmonic of 1 MHz is 1 MHz).
The thing which will cause more trouble than harmonics is spurious emissions which happen at frequencies in the broadcast band. These can cause more trouble because they can intefere with your neighbors' reception whereas harmonics rarely will.
Cell phone transmitters are designed to produce less than -60 dBc for comparison. This is one millionth of the typical carrier power of 24 watts per channel at a cell site.
Neil