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hi,
hi,
i have been looking through the ARRL antenna book and handbook for antenna designs and want to try some out for part 15. theres a lot of great looking mobile antennas that look suitable for part 15 operation. however, they are pretty much all designed to be connected to a 50ohm transmission line.
as i understand it the amt3000 is not matched to 50ohms but instead some other high impedance. i know that the pi-network is switchable but is there a setting on it that matches 50ohms impedance? if not, do any of you know the load impedance of the pi-network?
i was thinking i could add a simple L-Network to match the output impedance of the pi-network to 50ohms so that i could try out some 160m shortened vertical designs from the ARRL antenna book.
thanks!
Total posts : 45366Members with more technical education will be more accurate with an answer, but I have a guess why your idea is not sound.
The 50-ohm antennas you are looking at are designed for much higher frequencies, and if you match them down to a 50-ohm output most of your very tiny power will be lost in the matching circuit.
The AMT5000 is designed in particular for a 3-meter antenna, in other words the transmitter is custom-made for part 15 application, with a minimum of lost energy in pointless impedance conversions.
If I’m wrong I don’t know it.
Total posts : 45366Except that I also would like to know how to match the AMT5000 to 50-ohms for a different reason, namely to try it with a carrier current coupler that requires a 50-ohm source.
My idea might be silly and pointless, but I also don’t know that.
Total posts : 45366sorry i wasnt clear in my post. i was looking at mobile antenna designs for 160m (1.8MHz) which i believe would be suitable for a broadcast band part15 station.
using the amt3000 for carrier current is another reason to produce a matching network for 50ohms.
L-Networks are extremely simple to make using many calculators such as this one:
http://www.changpuak.ch/electronics/calc_18.phpif we would just need to know a little bit about the current pi-network on the amt3000.
Total posts : 453661.8mHz mobile is definitely worth matching to a part 15 use.
Maybe ask PhilB at sstran.com
For 100mW to work with carrier current the power line would need to be very sensitive at the operating frequency of the transmitter…. but as RFB tells it, that can happen.
OH, and a belated “Woops,” over the fact that my original reply mentioned the AMT5000 while you had mentioned the AMT3000.
Between us we need the answer for each of these transmitters.
Total posts : 45366Ahh..100mW CC. Well here is the deal on that.
In my testing of my CC system using 100mW transmitters, the 50 ohm input of the coupler really needs the source to also be 50 ohms. The line side is really irrelevant when it comes to the input side. You see..the coupler isolates the transmitter with a 3000 to 1 ratio. So even with a .1 ohm loading resistance on the line side of the coupler, the transmitter will always see 50 ohms and never break a sweat over the load impedance on the line side, be it greater than 50 ohms or less than 1 ohm.
Now here is the other side of that coin. In testing the low power transmitters, the best result came from using a transmitter configured for 50 ohms or even 75 ohms, such as the Talking House V5 unit, which has an external antenna connector and that output circuitry is configured for 75 ohms. Close enough to the 50 ohm standard of the input for the coupler to drive a 100mW signal through it and effectively couple that 100mW of signal onto the line sides of the coupler..again be it .1 ohm or anything else..even wide open, the transmitter still sees the input load impedance of the coupler. This is why the TH V5 unit can send it’s 100mW (actually its about 135mW) through the coupler and inductively couple onto the power lines with incredible efficiency and send that small signal down the lines a good distance.
My test with the TH V5 into the TCU-30 coupler puts the signal out to about a 4 block coverage area, and that is coupling to the neutral of the power grid, and with an isolated Earth ground, the line side impedance..or neutral in this case is just above 75 ohms. Almost a perfect load match on both sides of the coupler.
I do not have any test results using a transmitter that is configured for a higher impedance load like an AMT 3K or 5K. But if either of those units can be configured to load up effectively to 50 ohms, they should perform just as well if not better than the TH V5 unit.
And if those two units can be configured for 50 ohms output, it should not be any problem matching them up to a 160m antenna system with a little help from an L network.
As long as that 160m antenna is 10 feet long or less, the legal requirement will also be met.
RFB
Total posts : 45366I only have a part time brain, but it just did some work and asked a very smart question….
For the AMT3000 and the AMT5000 what would be a proper dummy load?
good one, huh?
Total posts : 45366As you guys may remember, there is a
Part 15 station that operates out of
the back of a truck. The guy parks the
truck at a pond in the middle of the
town, and plays tapes all day long that
were recorded in his studio. Then he signs off
at sunset. This is in Connecticut. I believe he
uses a Range Master which is mounted on the
back of the truck. He parks the truck in a
somewhat remote location and puts down a
temporary ground system. He gets about
1/3 of a mile coverage radius, which I think
is very good considering the set-up. Oh – he does
this just on Saturdays. But I think it’s a great approach.What makes this interesting is that, if a Part 15
AM transmitter supplied output at 50 ohms,
it could be used with a 160 meter mobile antenna.
(For either mobile or operation at home.)
However, the antenna would have to be modified,
and I don’t know how hard that would be. And, yes,
the antenna would have to be at least shorter than
3 meters in length.Since I am a ham, I have heard some stories about
mobile operation in the 1.8 to 2.0 MHz range.
I know people with 5 to 10 watts have talked to
other hams a few hundred miles away, but of
course this was for ssb or cw. However, I heard
about an 8 watt 160 mobile AM set-up that was
heard far across the state of Connecticut.I suppose it really all comes down to conditions.
Anyway, the bottom line to me is, any 50 ohm Part
15 AM transmitter might be made to work
with some of these mobile antennas. This could open
up a new world for us.I am not in the position to research a matching network
to lower the SS-Tran’s output impedance. But it is an
interesting problem, and maybe somebody will have
an idea.Bruce, DOGRADIO STUDIO 2
P.S. I think the output impedance of the SS-Tran AMT-3000
is around 800 ohms(?) But I have no idea what a
suitable dummy load would be like.
Total posts : 45366They call it a “screw driver antenna” amoung other things.
The loading coil has a movable contact on a nut on a threaded rod up through the loading coil. A long whip is mounted on the top of the loading coil.
To tune the antenna, a small motor drives the screw which causes the movable contact to work its way along the loading coil to adjust the amount of loading coil needed to tune the system.
Expensive item…
Total posts : 45366Yup! Maybe someday, somebody can try that!.
Best Wishes,
Bruce, DOGRADIO STUDIO 2
Total posts : 45366The answer lies on the AMT3000 website:
http://www.sstran.com/pages/sstran_amt3000mods.html
Total posts : 45366I remember that mod from the manual for AMT3000, and I did it to one of my two such transmitters which is ready for a new loading coil that’s being made, but here’s the thing…..
It doesn’t actually say what the impedance is when it’s “low.” Is it 50-ohms? Is it 10-ohms? is it 125-ohms?
Total posts : 45366From the description…appears that the removal of the 820ohm resistor, setting all the switches to on and the two caps, the load impedance is “swapped” over to the external loading coil.
From instructions” “The AMT3000 internal inductors are bypassed, and instead, the external base-loading coil provides the required inductance.”
So basically the final impedance is dependent upon the impedance of the external loading coil.
Does that mean it’s 50 ohms? If the loading coil input impedance is 50 ohms…then it’s 50 ohms.
I could be wrong about that, I don’t have the schematic to know for sure. But just going by the mod instructions that seems to be the case. Which is a GOOD thing if it is.
That means once the mod is done, then the output impedance of the transmitter is “wide open” to whatever load impedance you wish it to be based on what you connect to that output.
That is actually better than trying to build a coil to have a 50 ohm input impedance. If your loading coil is 200 ohms, once connected to the TX the load impedance becomes 200 ohms.
If it is connected to a 50 ohm load, then its 50 ohms.Again I could be wrong about that. But it makes sense to have it set up like that so the job of interfacing to an external loading coil is that much easier and gives a better RF transfer efficiency, since the output of the TX will always be determined by what load is connected to it.
RFB
Total posts : 45366RFB, i think it means that you are substituting the external coil for the inductors in the pi-network. this doesn’t mean the transmitter is “wide-open.” there is still a pi-network matching the source impedance to the load impedance, whatever those impedance values are.
my amt-3000 is on my roof currently and i dont have time to take it down until this weekend. would anyone with an amt-3000 modded for the baseloaded antenna be able to test it with a 50ohm dummy load and an oscilloscope and report back the rms voltage?
if it is ~2.24Vrms then the transmitter is able to pass its full power output into the 50ohm dummy load.
this might indicate we can use the amt-3000 with 160m ham antennas designed for 50ohm feedlines.
i have a 160m 10ft top-loaded vertical design i found which i really want to try building.
Total posts : 45366A couple of years back this was discussed in detail here but for the moment I cant’ find it with a search because I haven’t hit on the keywords.
Essentially, the AMT3000 when configured for the external base coil loaded antenna uses part of the loading coil inductance to achieve an impedance match at about 800 ohms at the output of the transmitter. This happens because the coil/antenna is not tuned to resonance but rather slightly to the inductive side of resonance. It is this “leftover” inductance which combines with the output network in the AMT3000 that transforms the low antenna Z up to 800 ohms for the transmitter.
I will try later to find the discussion in our archives.
ETA Found it here: http://part15.us/node/883#comment-4258
If you are interested, scroll up to view the entire thread.
Neil
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