hi guys,
i finally got a chance to test out radio8z's method. i used a radioshack 100uH rf choke and two 3ft lengths of 18awg stranded copper wire with a hp 54600b oscilloscope.
here are a couple photos. i took them after putting away the oscilloscope but you can see the dipole and choke in relation to my transmitter setup. 
http://i.imgur.com/xIr2s.jpg
with the amt3000 turned off i was seeing about 400mV on my scope. with it turned on and the antenna tuned to 10.5v peak the scope was reading between 680mV and 720mV. i tried adjusting the antenna off resonance to see what would happen but the scope tended to stay in that range unless i dropped the antenna way out of tune (1 or 2v peak reported about 500mV on the oscilloscope).
my conclusion is that the scope is not an ideal field strength reader. its really difficult to see a peak on a scope. i would imagine that an analog meter would be much easier to read.
Thanks for the report on your experiment. Try it with more separation between the transmit antenna and the monitoring antenna. It appears from your pictures that they are too close together and you are getting more capacitive and inductive coupling than you should and this is swamping the actual radiated field energy effect. Separate them as much as practical to get a usable reading on the scope.
Mine is 6 feet away from the transmit antenna and is at right angles to it to minimize this coupling. There is a definite peak at resonance. You are probably right about an analog meter being easier to read than the scope. Here's an idea you might try: Connect a diode to one side of the coil, a capacitor (.01uf for example) from the other end of the diode with the other cap lead connected to the other side of the coil and connect a DVM across the capacitor. The DVM is much more sensitive than a D'Arsonval movement and may have the sensitivity required to give a meaningful reading. Use a 1N34A or other germanium diode if possible. I haven't tried this so if you do let us know what you find.
Neil
its too dark out to take photos this time.
i used a mystery diode i believe to be germanium. it has glass body and one green stripe on the cathode side. i put the 104 capacitor on the anode side of the diode and put them both between the two leads of the coil.
i taped the field strength meter's antenna in a spot 1ft off the ground and about 8ft from the amt3000 antenna.
i used my fluke 117 multimeter set to the mV DC setting. am i right to have it on the DC setting?
with the transmitter turned off it read 0.5mV. with the transmitter turned on and the coil way out of tune it also reads 0.5mV. the tapping the coil at the point i know to give the highest peak voltage (usually 10.5V, but it varies throughout the day) caused the meter to fluctuate between 3.6 and 4.1mV. most of the time it was at 3.6mV. the points around this tap (which i believe drop the peak voltage down to around 8 or 9V) showed 2mV on the meter.
conclusion: this method is much easier to read than the oscilloscope method (and it is way more convenient than lugging around a huge oscilloscope). however it is annoying to not be able to verify the peak voltage at the same time (i only have one multimeter).
i am curious why the values picked up would be so different (3.6mV vs 700mV). its too dark outside to do any more testing tonight but tomorrow i am going to try elevating the field strength meter's antenna to be about parallel with the antenna. currently it is slightly below the antenna.
ps- i have a question about the peak voltage reading from the probe points on the amt3000. what exactly is that measurement called and what is it actually measuring? all i know is that higher is better and that it has something to do with resonance and how much power is being coupled to the antenna.
edit: fixed some grammar.
Using the tuned loop antenna into the o'scope has plenty of sensitivty. And yes, the tuning cap gives me obvious sharp tuning to help eliminate other RF.
My antenna is outside, the loop antenna inside.
mram that sounds pretty cool. my scope is an hp54600B which is rather large and heavy. its a great scope but it is not very convenient to use for testing field strength. 🙁
today i went out to my local electronics shop and picked up a 50uA ammeter to put together basic field strength meter from this site: http://www.utaharc.org/rptr/wdr_fsm1.html
i threw it together on a breadboard and used a 3ft length of wire as an antenna. i placed the whole thing about 6feet from my amt3000 and it seemed to work okay. i had to turn the sensitivity all the way up and even then the meter peaked around 5uA but it was definitely working. pretty exciting stuff. 😀
i'll post a photo once i have a nice enclosure to put the whole thing in.
Here's what I used with a sensitive digital multimeter:
http://www.electro-tech-online.com/electronic-projects/58-simple-field-strength-meter.html
Don't tune with modulation ...
Unless by stroke of luck, the taps won't bring you to final resonance.
Once you have it as high as you can go with taps, you need to do a final length adjustment. In the antenna type you've built, it should have approx 3" cross slots in the short section attached to the top of the loading coil. It should be 1" dia. copper pipe section, and of course a 3/4" pipe for the main section.
With the top section slid inside the short section, I used a pair of hose clamps to tighten it, but only enough to hold it in position.
Then you have to work it up and down while watching the meter (or O-scope), stepping at least 3-4 ft. away from the antenna before each check. If it gets less than the reading with just the tap, work it the other way and it should rise to a point where the reading starts to get less again ... and visa versa if it is more than the tap reading. Continue the process until you get the reading as high as you can. When you have it, your system is in resonance. Lock it in position by tightening the hose clamps.
It took me quite awhile to get mine adjusted. Patience is required. HTH ...
P.S. - If you want you can stick a low range air cap, say 0-10 pF, to make small day-to-day adjustments to compensate for changes in atmospherics and ground conductivity. I don't (I have great ground), but I'm pretty sure there are others who do.
i'm aware of the antenna length adjustments and have definitely been doing them. i've noticed that at different times of the day the peak voltage shifts between 10.5 and 13v.
when using a variable capacitor, where do you wire it in the circuit?
"when using a variable capacitor, where do you wire it in the circuit?"
In the transmission line between the TX and the loading coil. Use a large-style multi-blade metal air variable cap.
oh cool. i've been wanting an excuse to buy some of those. they look so cool!
can you recommend a capacitance value to look for?