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Howdy,
So how about this?:All in testing stage …
Howdy,
So how about this?:All in testing stage …
I have a little Spitfire AM TX running on 1630 kHz. At the moment, I’m trying a marine VHF whip (101″) mounted on two 8 ft marine mast sections. I’m using a coil of 2″ white PVC pipe close wound with 18 gauge insulated stranded wire (which hopefully gives enough separation for less transfer between turns). It needs to be elevated due to surrounding buildings and across the field to the main downtown area I want to try and reach.
I’m the live signals from Garageband (Apple computer) with software limiter, multi-band compression, and SoundSoap.
The signal is pretty clean, same one I’m simulcasting on the web, but it’s still pretty weak on real radio. The only ground the TX has is from the audio input cables and power supply (2-prong which has no separate ground wire) and not the ground wire connection at the TX. Therefore the only ground it has is the ground for the building where I have a studio in a unit, maybe 80 ft. or more from the building’s earth grid.
So here’s what I’m thinking: Instead of using the TX’ own separate ground connection, keep it’s ground as is, and build a radial system at the foot of the antenna which is also connected to the building’s ground.
Will this work at all? Will the main grid prevent the radials from working with my antenna?
TIA,
Ken N.
Total posts : 45366What kind of coverage are you getting now or rather how far is it broadcasting just like it is now?
I ask this to get a rough idea what your station is doing right now.
Glad to hear someone is using the Spitfire TX.How do you like it?
Total posts : 45366Very rough estimate: With the antenna lying horizontal across a couple of wood railings outside my shop entrance, about 230′ across a parking lot and into a block building with the garage door open, to what looks like a late ’80’s vintage AM/FM stereo receiver.
By the time it got there, it was still plainly distinguishable, but with a chunk of fuzzy static.
Run straight from iTunes (no other processing) on an iMac through an old-model Griffin iMic (small USB interface). No ground wire other than the shields on the audio connections to the TX, plus however the two-wire power supply is grounded.
The Spitfire audio is good, certainly intelligible, but music is brown, i.e., a little muddy and a pinch noisy to my car radio 20 ft. away. It’s there, but it needs some pre-broadcast work to clean it up and get some compression and EQ together, Don’t know about that yet.
Total posts : 45366Before I spend a bunch of work time and money, I’d still like to know if setting radials from the building’s electrical ground will work, as opposed to from the TX’ own ground connection post.
If the TX’ chassis is grounded via the audio inputs and power supply, it actually seems to me that setting a second deep ground connection directly to the TX attaches it to the same ground as the building as well. If there is any leakage anywhere (it’s an old building) I’d think there could be a potential across the chassis’ of the TX and the audio gear, which would cause noise, maybe even a partial ground loop. Right now, because I’m not using the TX’ ground directly (although I am trying to use it to measure the difference between it and the opposite end of the coil … is that wrong? … I don’t get a reading at the 50uA switch scale), there is little such noise @ car radio 20 ft away.
If the chassis of the TX is grounded to the building, what will happen if I attach ground radials below the TX, but no ground lead from the TX … IOW, still using just the audio and PS connections for grounding?
I could always use he TX ground connection to a lightning gap and then to the ground as an arrestor.
Total posts : 45366I am not being sarcastic, but trying to be helpful. Look at AM radios stations. The ground radial are at the base of the antenna for a reason. 🙂
Total posts : 45366Hi Ken,
Though your transmitter is “grounded through the audio leads” it may not be so to RF. For low frequencies, it certainly is but RF is different due to stray inductance. Try to develop a picture that an RF ground looks different from a low frequency ground. Just a right turn in a ground conductor can introduce quite an impedance to the flow of RF current. Certainly a tortuous path running many tens of feet through the power system of a building is not going to provide an effective RF ground.
If you are not willing nor able to mount your transmitter above ground radials and grounded thereto, then try to ground your transmitter to the electrical power stake or a cold water pipe using the shortest and straightest path possible. Even the cold water pipe may not be good since often buildings with metal pipes are fed with plastic water mains and the pipes are actually bonded to the electrical ground stake.
You are asking the right questions but you need to think differently about RF circuits. They are not the same as low frequency power or audio circuits. You commented:
“Right now, because I’m not using the TX’ ground directly (although I am trying to use it to measure the difference between it and the opposite end of the coil … is that wrong? … I don’t get a reading at the 50uA switch scale), there is little such noise @ car radio 20 ft away.”
From this it appears that you are trying to use a multimeter to make RF measurements. This rarely works because the frequency response of multimeters is not adequate for radio frequency testing and also because the multimeter’s input capacitance and lead inductance will detune most RF circuits. A multimeter set on the 50 uA scale will also insert several hundred ohms resistance in series with the circuit.
There is no harm in experimenting but if you know some of the guiding principles (such as short, straight ground connections) you can avoid a lot of wasted effort.
Neil
Total posts : 45366Of course … I was assuming the radials to be at the base of the antenna mast … just that they would be connected to the building’s ground instead of the TX’ ground post and still be the same ground it has from the shield on the audio cable. That would eliminate the need for a long ground lead from the TX up on the mast, hopefully remaining in Part 15 compliance.
Total posts : 45366Thanks Neil,
Yes, I understand what you are saying. My problem is that I want to get into the main town area and the ferry terminal about .5 mile, but there are buildings, cars, pavement, etc. between here and there. I figure I need to get it as high as I can within legal limits her (27 ft. from the ground or 15 ft. above a roof … in town). I’m winding a Manteca MAgnum style base loading coil today.
So, the problem is adequate grounding for the TX within Part 15 regulations. What if I run the ground using the main wire of a shielded cable … sort opposite the way you would send a signal from an indoor TX to an antenna if it were a licensed project, but with nothing on the shield? I understand proximity effect of an RF ground in that scenario, but the questions are:
1) Will blank shielding negate the ground RF enough to stay in compliance?
2) Will there again be too much resistance if such a ground cable is, say, 18 ft. (two sections of 8 ft. marine mast plus 1 ft. of mounting base?
3) If not, how can I get the RF ground from the antenna on the mast to a ground system at the base while staying in compliance?
The total antenna height will be the town’s legal limit: 27 ft. … 16 ft. for mast and antenna, about 1 ft. for inline loading coil, 9 ft. for antenna, and 1 ft. for base mount. It upper mast will be attached to a building eve rafter extension, cables come out through a vent under the eve.
I plan to get the audio signal from my studio mixer to the mast/TX using balanced XLR cable with reverse adapter (50 ft. XLR cable -> double-female XLR adapter -> XLRM -> 1/4″ stereo adapter -> 1/4″ TRSF stereo splitter -> dual RCA -> TX audio inputs.
But I would really rather run a fiberoptic cable. If I can get down to Boeing Surplus someday, maybe I can find the components for that on the cheap.
I think the 15 vdc power wire from the power supply should make it from the attic vent to the mast and TX.
Gotta go digitize some tapes and burn CDs now … I’ll send a link to pix when I get things ready to mount.
Total posts : 45366Ken,
I hope I understand what you are asking so here goes:
1) Will blank shielding negate the ground RF enough to stay in compliance?
No. The shield will not prevent radiation. This only happens when the RF current through the inner conductor is equal and opposite that through the shield which is not the case here.
2) Will there again be too much resistance if such a ground cable is, say, 18 ft. (two sections of 8 ft. marine mast plus 1 ft. of mounting base?
No. If it is straight run and the mast sections are bonded together well resistance is not the issue.
3) If not, how can I get the RF ground from the antenna on the mast to a ground system at the base while staying in compliance?
This has been discussed ad infinitum here and on other boards, and I am not going to comment other than by stating that any ground lead between the transmitter and the earth ground will radiate a signal and contribute to the field strength.
Unlike with FM, for AM height is not necessarily your friend since the AM waves travel along the air/ground interface. Notice that AM broadcast stations do not elevate their “sticks”.
Neil
Total posts : 45366Ken,
Are you saying there are NO antennas over 27 feet in the city limits? No government, etc..
Total posts : 45366Hi John,
No, I’m saying there are no antennas from the ground higher than 27FT. We can go 15FT from a rooftop
Total posts : 45366Hi John,
FYI for this group: Look at these to get an idea of where I am:
http://www.islandcam.com … my boat is to the left (West side) of the marina, out of frame.Google Map of Friday Harbor
… If you zoom in 4 clicks, you’ll see a street named Malcolm St., just South of the main “downtown” area (downtown here means 4 blocks ;)). The street heading South doesn’t actually exist, but if you were to make a short street North from about the same area, which isn’t shown (go figure), you’d be driving into the small business park where my studio is. On the downtown side of the plot back of the building I’m in, there is a fairly large field not owned by my landlord. I may be able to get away with burying some chicken wire there under my antenna, though. Or at least a couple of 50FT wires parallel to the building, East and West from the base.Friday Harbor, WA is an incorporated Town of about 1600 people, and is the county seat for San Juan County … which is an archipelago of small islands in the Puget Sound. It’s on San Juan Island, one of the four ferry-served islands in the county (there are actually 172 islands in the archipelago, many tiny ones, most uninhabited, and some of which go beneath the surface at high tide ;))
The “government” has small towers for Sheriff and ES on the courthouse roof. And Washington State Ferries has some on their little building. Plus the cell towers and CenturyTel trans’ on other bldgs. None of them are over 15FT above their respective roofs.
On another island in SJ County:
Lopez Island’s 18 watt LPFM Community Radio station (on the air for 1.5 years):
http://www.kloi.org
FCC query shows:
call=KLOI-LPBy contrast, there is a group trying to make a 100 watt station here on SJ Island … KSJU 91.9 LPFM. FCC query shows:
KSJU 91.9 LPFMBut the BOD has chosen not to join Prometheus, and doesn’t seem to want to follow advice from other successful active LPFM stations. They keep their meetings closed. They are having trouble raising funds (no surprise to me). There is a potential for a great LPFM station, but the way they appear to be going about it, I think it might be another 2+ years.
But in the meantime, I think the people are ready for some radio that can serve the community’s interests right now. I just want to get something viable and supportable up and running.
I think I can have a decent web presence and provide some local service programming, plus a platform for potential programmers to develop more content. That way, when (and if) KSJU gets on the air, they can have ready-made programming to flop over which is fine with me, I’ll be open-handed … I really want to see it be successful. But presently they don’t have anything tangible to offer the community … I figure I can bridge that gap, if they can see their way clear to accept it. If I can get solidly on the air, I bet I can help them raise funds faster than their current ideas … which aren’t working.
Total posts : 45366Maybe because I bracketed them?
Total posts : 45366Yes, using brackets around the link url will cause the link to not show up. I added full html links to your post and as the original author, you can click the “edit” option to see how that looks in the post bucket.
To be fair, the process has changed as we use different versions of the content management system. At one time BBS code worked here, and some really old content still needs to have that cleaned up by me.
Currently, just pasting the url info will show up as a link, like this
http://part15.us/node/2093#comment-7146
is the url to this commentOr, you can us full HTML by selecting that from the Input format option in the posting user interface.
Total posts : 45366Wow. Guess what, FCC PRB-1 would exempt you from that limitation if you possessed an amateur radio license. That being said, given the locale, you should be able to do jest fine with AM part 15.
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