sorry to make a whole post about this but i found an old thread ( http://part15.us/node/1846 ) where radio8z mentions using an oscilloscope as a field strength meter. he didn't actually say how to do it in the thread and i didn't find any other threads that talk about this. and so i am asking, how can i go about doing this?
i want to do this so that i can better check the tuning of my loaded antenna for my amt3000.
i was also wondering if there was a way to determine the impedance of antenna so that i can measure the output wattage?
i hooked up my function generator and oscilloscope to a dummy load and set the function generator to put at 2.25V RMS and then replaced the dummy load with my sstran style antenna. i attached my probe to the copper pipe and tried putting the generator through the different tap points while cycling through different frequencies. the highest rms voltage i was able to get at 1.7mhz was 6V RMS when connected to the top most tap point and 3v at the bottom tap point. at frequencies around 1.2MhZ to 1.4MHZ i was able to get over 100V RMS if i picked the right tap point!
is this a proper way to test an antenna's resonance?
if so does this mean that my antenna is best for frequencies around 1.2-1.4MHZ? if i made a coil with more turns would i get a higher rms voltage at 1.7mhz (my desired frequency band)?
Here's how I do it. I use two pieces of wire about 3 feet long, one connected to the probe ground and the other to the tip. To eliminate 60 Hz hum I connect a RF choke from tip to ground. Positioning these wires stretched out to make a dipole about 6 feet from the transmit antenna yields about 50 mV RF signal on the scope. Just adjust the transmitter and or antenna for a peak reading.
About any configuration of lead wires will work but keep them away from the transmit antenna so you don't detune the system. The voltage you get depends on a lot of things and the 50 mV I mentioned is only a typical reading for my setup.
When adjusting an antenna system for resonance you do not want to connect any test equipment to the antenna. I have found the presence of my body within 3 feet of the antenna will measurably detune it.
Neil
thanks for the explanation. i am going to test this tomorrow.
do you have any comments on what happened when i hooked my antenna up to my function generator and oscilloscope?
The connection point from the coil to the copper pipe or other radiator is a very high impedance point and any connection made here for measurement will load the antenna and the readings will be meaningless. Even a 10X scope probe will present a resistive load which will pull the voltage down and a capacitive load which will change the resonant frequency of the antenna.
Using the measured Z of my antenna system and the measured input current at the base of the loading coil the voltage at the bottom of the radiator is calculated to be approximately 100 Volts with 1.5 Vrms at the coil input. This implies that the 1.5 Vrms is transformed up to approximately 100 volts by the coil/radiator combination which is to be expected since this is a series resonant system. This voltage for other antenna systems can vary greatly since the ground loss and the coil Q affect this voltage.
I have found that a very good way to assess what is happening with the SSTRAN type antenna is to measure the feed voltage, current, and phase angle. This makes adjustment relatively easy since at resonance the phase angle is 0 and if not resonant then the angle indicates which side of resonance the antenna system is set so the appropriate adjustment can be made.
For the current measurement I use a home made RF current transformer which displays the current waveform on the scope. I'll try to put up the details of this in the next few days as a blog post.
If a SSTRAN AMT-3000 transmitter is used then the antenna coil inductance is set off resonance so there is a "residual" inductance which functions with the transmitter internal parallel capacitance to transform the antenna impedance up to about 800 ohms required by the transmitter final. If this is the case it is best to tune the antenna according to the SSTRAN instructions rather than by field strength.
Neil
thanks for the tips. i figured the scope with a 10x probe would interfere with the antenna but assumed it would drop the voltage somewhat but still give ballpark measurements of resonance. i didn't think it would actually be adjusting the resonant frequency of the antenna system. thats interesting.
when you talk about measuring feed voltage are you measuring ac or dc? do i just use my digital multimeter to do this or do i need some other gear? what about phase angle, what do i use to measure that?
i know that sstran offers a specific way to tune the amt3000 but i plan to experiment with other transmitter kits—and eventually try doing some homebrew stuff—and i want to learn more general methods of tuning antenna systems.
what about swr meters? are there any for medium wave? would one be useful for tuning a part15 antenna system?
an SWR meter would be useless in part 15 operations.
Mainly because there is no way to actually get the antenna resonant.
Any tuning is done to get the transmitter matched to the antenna.
i thought that adjusting the tap points on the loading coil was a process of tuning the antenna to resonance?
radio8z, as far as the field strength meter. . would this rf choke be appropriate? http://www.radioshack.com/product/index.jsp?productId=2103978
and do i want to have the transmitter modulating when i do the field strength test?
The choke should work. The one I used came from my junk box and I don't know its value.
It is easier to read the scope pattern if the signal is not modulated. Also, if you want to check the modulation with the scope a sweep setting of 1 or 2 milliseconds/division allows you to view the modulation envelope.
Neil
I connect my scope to a homemade tuneable loop antenna.
The main loop is about 10 turns on a wood crossbar about 20 inches across. The main loop has an AM BCB air variable capacitor to resonate the loop. A second one turn pickup loop connects to the o'scope input.
Seems to work pretty good.
out of curiosity, what do you guys think of the "conventional" field strength meter design on this page? this looks like something i could put together really easily.
the page says that for signals under 1watt you have to be under 10ft away from the antenna. is that a problem if it is only being used to tune an antenna?
I didn't see a link to the meter you mentioned. Generally, a field strength meter usable at part 15 AM signal levels will require at least a one transistor amplifier. Those that use just a diode and a meter might work if you are close to the antenna but the trouble with this is being close to the antenna will detune it.
Neil
oh woops i forgot to include the link. http://www.utaharc.org/rptr/wdr_fsm1.html
hmm. maybe that simple one wont work for part15. do you think the more complex one using an op-amp on this page would work?
While some of the comments in this thread have talked about having home-brew tuning circuits or such in connection with their oscilloscope field strength hookup, I have a long held question about field strength measuring in general....
Isn't it very crucial that the field strength reader, be it a meter or a scope, be tuned to the desired frequency?
If not, it seems to me, there could be many nearby frequencies strong enough to "shoulder their way in" and get registered.
weird. my post didnt go through for some reason.
here is the link i forgot to include earlier: http://www.utaharc.org/rptr/wdr_fsm1.html
it includes a simple diode ammeter design and a more complex one using an op-amp.
if the simple design isn't suitable for part 15, would the op-amp design be sensitive enough?