Typical frequency used with SSTRAN 3000

I guess most people must operate AMT-3000s (or any other

Part 15 transmitters) from 1610 to 1700 kHz.  It's less crowded up

there and the short permitted antenna works best there.  (I know you

know this but I figured I would mention it anyway.  Here in the northeast,

New England, or there abouts - - so many Part 15 set-ups were on 1700.

UNTIL an AM station in NY (actually 112 miles from me because I checked)

moved from 1300 kHz (500 watts day and either very low power or nothing

at all at night) to 1700 10 kW day, 1 kW night.  Even though I wasn't crazy about

1700 being lost - I had great hopes for the station (WRCR, Ramapo, NY) because

they were involved with  the community and had very cool local programming.

Alas - - WRCR had to go to brokered programming - - probably to keep money

in the bank.  Last I heard their original programming was streaming on the internet

but it's been more than a year since then and I have not checked this lately.

Here in Hartford there is a foreign language pirate running on 1700 with a blasted

messed up over modulated garble extending up beyond 1710 and below 1690.

This beast has been overlooked by everybody and will probably

be on the air forever.

My Part 15 experiments used to be ON 1700 or 1690 but lately

I have been on 1590 just yardcasting through a bunch of 

antique radios (many of which don't cover the "X band.")

I am thinking of going back to carrier current but thst's another

thread.

Brooce, Part 15 Hartford

AB1AW got my interest by saying:  "I am working on modifications to increase the effective power output (within part 15 rules) and would like to share with others."

The only modification I know of for the AMT3000 is to use a hand-made loading coil with a few circuit mods instructed by the SSTran website. Is that what you are doing?

Or, and now things might get real interesting, is there another way to boost the strength of the transmitter within part 15 rules?

Used to be I think the majority of Part 15 was on 1610/1620.

I use 1670 in North Texas, 1650 as well when I was in the Dallas area.

Carl,

The SSTRAN 3000 has a potentiial maximum power output of 83 milliwatts into a 1860 ohm load. This is the power level actually delivered to the antenna. It is not the 100 milliwatts input power (conforming to part 15 rules).

My goal is to get all of this power to an efficient antenna system. The best comprimise of efficiency vs practicality is a center loaded vertical. This sort of antenna has a feedpoint impedance of 30 to 50 ohms. I am redesigning the output stage of the SSTRAN 3000 (C5, L4, L5, L6, L7, C23) to match the 1860 ohm impedance to the center-loaded antenna.

I am also designing the modifications in such a manner to make it easier for others to do the same without having to wind special coils or do other hacks to the PC board.

The reason I asked about most-used frequency is that this modification will not have the broad tunning adjustment range of a non-modified SSTRAN 3000. I wish to choose a subset of the AM band most useful for other part 15 folks.

Mike

Thanks for the project description AB1AW Mike. Very novel and something no one else has thought about doing.

Trouble with the "most used frequency" is, and this may already have been said, that in each location we are stuck using a frequency that is blank in that location.

Where I am the top of the dial, which is the most effecient frequency area due to short antenna, is mostly open because no local stations transmit above 1600kHz.

I use 1680 because 1690 from Chicago is somewhat strong here and 1700 has a 2nd harmonic from our local at 850.

By looking at radio-locator.com you can see where all the X-band stations are around the country, and some places have T.I.S. (traffic) stations on the band, so there is no single "most used frequency".

RE Reply 5:  ... The best comprimise of efficiency vs practicality is a center loaded vertical. This sort of antenna has a feedpoint impedance of 30 to 50 ohms.  I am redesigning the output stage of the SSTRAN 3000 (C5, L4, L5, L6, L7, C23) to match the 1860 ohm impedance to the center-loaded antenna.

Just to note that the impedance across the feedpoint terminals at the base of a 3-meter, vertical monopole and the path to r-f ground includes both a real, and a reactive term. 

The real term is comprised of the radiation resistance of the monopole (about 0.1 Ω in this case) plus the resistance in the path to r-f ground plus the r-f resistance of the loading coil needed to resonate that antenna system.

The reactive term is defined by the capacitance between the 3-m monopole and the earth surrounding it out to about 0.5 wavelengths -- which for this setup might be on the order of 3,000 Ω.  Commonly a loading coil is inserted in the monopole to offset the reactance of its capacitance to the earth, which maximizes the power that the transmitter can deliver to that antenna system.

But neglecting the losses in the r-f ground path and the loading coil, this antenna has a feedpoint impedance of about 0.1 -j 3000 Ω.  ==> Only the radiation resistance component of the real term (0.1 Ω) can produce useful r-f radiation. <==

If the resistive loss in the r-f ground path is, say 60 ohms (as it might be when using a single ground rod buried in soil of average conductivity), then without the presence and effects of the loading coil, the complex impedance across the feedpoint will be 60.1 -j 3000 Ω.

A suitable loading coil can offset the capacitive reactance of this short radiator and resonate this antenna system, so as to maximize the power it can radiate.  However that coil will have an r-f loss adding to the other real losses in that antenna system, which can range from maybe 20 Ω for a coil at the base of the monopole to maybe 30 Ω for a coil at the center of the monopole.

So when all the numbers are accounted for, a center-loaded Part 15 AM whip might have a feedpoint impedance of about 90 -j0 Ω at the upper end of the AM broadcast band.

I am also designing the modifications in such a manner to make it easier for others to do the same without having to wind special coils or do other hacks to the PC board. ...

However a ~3-meter monopole antenna system for Part 15 AM _NEEDS_ a "special coil" in order to resonate it, and maximize the r-f energy it will radiate.

Without that coil, the 50 Ω SWR at the feedpoint impedance of 60.1 -j 3000 Ω for this system is about 3000:1, and it would produce almost no useful r-f radiation.

Carl,

Thanks for the tip about radio locator. I was hoping that it had a feature to find vacant AM channels, but it has that feature only for FM channels. In any case, I've already found the vacant AM channels in N.E. using the FCC database. From 1610 KHz and up are clear in my neck of the woods, with a smattering of other frequencies elsewhere in the band.  I am going to suspect that this is typical elsewhere in the U.S.

Also, 570 KHz and 650 to 700 KHz are also clear/vacant. This is an enticement as groundwave propagation is significantly superior at these frequencies compared to 1600 KHz. I need to do some studies as to whether the decreased antenna efficiency at those lower frequencies is less than the increase in propagation capability.

Mike

Thanks Rich for the in depth look at the variables involved with transmitter antenna matching in the exceptional case of short antennas. I think I have a question to ask but I need to more closely read everything said on both sides to make sure it wasn't already addressed. I'll get back to it before this thread runs out.

To ab1lw here is a radio-locator technique that will give you more power to explore.

On the main page read down the list and click on "Advanced Search".

On the Advanced Search page enter a frequency in the box, let's say "1700".

You will be given a list of every radio station in the U.S., Mexico and Canada on 1700 kHz.

PhilB of SSTRAN makes it clear on his site and in the docs that the upper part of the AM band is best. He also states elsewhere on this board that other antenna mods are, in my words, mickey mouse engineering.

I'll be making that base loaded antenna soon. If you'll cover the copper, pvc and shipping I will wind you a coil when I'm done with my own. Don't forget about the board modification for the AMT 3000 if you use the base loaded antenna.

The SSTRAN 3000 can be modified to drive a 50 ohm antenna (non-reactive impedance) by making the following component changes:

Freq (KHz) 1440
R18 (ohms) 2700
L8 (uH) 1000
C23(pF) 220
L4 (uH) 0
L5 (uH) 0
L6 (uH) 22
L7 (uH) 22
C5 removed

Freq (KHz) 1640
R18 (ohms) 2700
L8 (uH) 1000
C23(pF) 180
L4 (uH) 10
L5 (uH) 10
L6 (uH) 22
L7 (uH) 0
C5 removed

Modifications are based on calculated transmitter efficiency of 85% at 100 milliwatts input drive with a 3825 ohm load at collector of Q5.

These modifications allow the transmitter to achieve near 100 milliwatts peak output into 50 ohms with 100% modulation driven by 1 KHz 300 mV audio signal.

Measurements made and conformed using the following test equipment:

Siglent SDS 1052DL digital oscilloscope and frequency counter.
MFJ-260C 50 ohm dummy load.
Fluke model 75 DVM
Lodestar SG-4160B signal generator
DX-370 receiver
Keuwlsoft audio function generator
Palstar ZM-30 digital antenna bridge
Sony Discman D-11 (playing Beach Boys Greatest Hits)

So are we saying that the unmodulated carrier is in the vicinety of 25 mW?

The PEP of a 100% modulated AM signal is 4x that of the unmodulated carrier.

Most part 15 transmitters have an unmodulated carrier in the vicinity 100 mW input power. This said 100 mW @ 100% modulation.

A transmitter w/ unmodulated carrier at 100 mW must necessarily reduce that carrier power when modulation is applied. Otherwise it exceeds FCC regs.