Test of TEN C. Crane Transmitters + Special Effects!

Thanks for doing these tests, a lot can be learned.

Mark

... The tests are all done with the receiving and transmitting antennas
 7 feet above ground as specified in the Potomac manual so that the proper antenna factor can be applied. ...

A very interesting set of experiments, Tim.  A fine effort, that.

But for further consideration we should note that the IB for the Potomac FIM-71 correctly is silent on the height above the earth of the radiation center of the transmitting antenna, the maximum power it radiates, its radiation pattern, and other propagation/reception factors responsible for its e-m fields captured by the receive antenna of an FIM-71 at any height above the earth.

Very interesting set of tests.  The tape measure test was particularly intriguing.

This is very true.  However, the same variables apply to the FCC agent lurking in your bushes. 

I've often wondered what the results would be working at various points around the transmitter, as well as at different heights. But that system is just to awkward to move around readily while testing.  Methinks for our purposes consistency in testing is most important -- do it the same way every time -- then at least results can be compared from test to test. 

TIB

That tape measure test was done because last year when I was testing the Decade CM-10, I had placed it in a front of the house second story window and was runing up and down 4 flights of stairs taking field strength readings in the front yard, then going upstairs and adding, moving, changing antennas, etc.  I discovered that if I moved things around on the desk -- even a pair of wire rim glasses -- that this would change field strength readings!

So I wanted to try it while I had things set up properly, and the only metal thing I had with me was the tape measure, so I set it on the transmitter, then got the idea to pull the tape out, etc...

I tell ya, this FM Part 15 stuff is freaky.  Darn near any change in surroundings, be it attached cables for power or audio, to metal in the immediate area, makes a change. And I'm sure the variance in field strength would have been different at different points around the transmitter at the same distance.  It's certainly not going to be a circular radiation pattern, no matter what the setup.

TIB

I also plan on doing basically the same test with Whole House 3.0 transmitters.  However, due to much higher cost I only have five of those new in the boxes waiting for me. 

If you recall my test of the WH 3.0 last year, new out of the box in the "USA" power setting, with the antenna horizontal, the Field strength was 1102 uV/m.  Not even close to legal.  Even with the antenna vertical it was 382 uV/m, also not legal. 

Was it a fluke?  Was it a bad transmitter with too much power output?  We shall see. That is, if I get to the tests before the weather goes to heck up here. 

TIB

From reply 5 above:

... I've often wondered what the results would be working at various points around the transmitter, as well as at different heights. ...

Insights and conclusions supporting the practical and proven performance of such configurations are possible without much physical effort or hardware cost when accurately modeling those systems using computer software such as NEC (Numerical Electromagnetics Code).

The FCC accepts such calculations as the proof they need to license AM broadcast stations.

Thanks, Tim, for the well done tests and writeup.  It is very interesting that you observed what you did with the nearby tape measure.

Question for Rich, would you be willing to run a NEC simulation of a transmitter model with and without a nearby parasitic element nearby to assess the effect on the overall field pattern?..  Such effects are well known, such as with Yagi antennas, but it would be interesting nonetheless.

Neil

Would such software include effects caused by tape measurers set on transmittgers, wire rim glasses on the desk, or movement of cables attached to the transmitter?  Seems to me these variables are so variable that inputting the data would be difficult. 

Is what you're saying basically that all field strength meters and other measuing devices be thrown away and we can just punch it all into a computer and call it good?  It would seem odd, as $14,000 Potomac FIM's are on backorder.  People seem to need them. 

TIB

Tim you brought up some very interesting things I really didn’t think of when using a transmitter and field strength and of course that equals range. Friends have told me to be careful of what I put around the transmitter because it can absorb the RF thus decreasing the range. But never has anyone ever mentioned certain items may increase field strength dramatically.

Imagine someone buying a legal certified transmitter at full or even in the case with the C. Crane less than full legal limit. Well they place it too close to a metal rod or pipe and the strength goes well above the limit. I wonder how that would hold up in court if a person replied “but I didn’t even think that my tape measure on top of my desk or glasses would have done all of this…

Now we have even more reason that it should be Watts, mW or something reasonable for FM not something less than 1 mW to measure.

Good work and this may even prove a case for us as hobbyists.

... run a NEC simulation of a transmitter model with and without a nearby parasitic element nearby to assess the effect on the overall field pattern? ... 

Below is a NEC plot comparing the free-space radiation patterns of a single dipole with and without a nearby parasitic radiator.  The driven radiator is shown on the right side of the array, having a red circle at its center (the feedpoint from the transmitter).

In this case, adding the parasitic radiator improved the peak gain of the dipole by about 4.34 dB, or about a 2.7X increase in radiated power in that direction.

Tim, I agree with you about the problems in setting up a valid model when the actual configuration and environment is not completely and accurately defined.  But this NEC example shows in general terms how nearby parasitic radiators can affect antenna radiation patterns.

An accurate field strength meter can be useful to measure the fields existing for a given set of conditions, but those exact measurements are unlikely to apply to different physical installations of "Part 15" systems.

Thanks Rich, that's a good plot, and I hear the effect all the time, walking through my room and my stereo gets static on an FM broadcast. You should form a test lab for part 15 products and make a business from it. You'd have no shortage of work.

Thanks for your comments.

You are doing the sort of things I want to do.

Peace, God bless