Whole House 3.0 FM Transmitter Field Tests

I spent nearly 3 hours out in an open field yesterday running tests on the Whole House 3.0 transmitter.

I purchased this transmitter at retail from a seller on Amazon, so what I tested was one as sold to the general public, not one handpicked by the maker or importer. 

Tests were taken with an accurately calibrated Potomac FIM-71, the same as the units used by the FCC in the field, and also used by most broadcast engineers who need to do work with FM broadcast stations.  The readings then were also taken with a ZTechnology R-506 which is used by a wide variety of RF engineers in dozens of applications and is also the "new" technology often used by the FCC for tracking down illegal operators because it can output to a laptop and also synchronize with a GPS unit and compute exact field strength from varying distances, etc. Needless to say I just use mine for manual field strength readings.  BTW, if you were to buy a new R-506 with its calibrated antenna the price is $13,000.  Each device was used with it's own calibrated half-wave dipole antenna.  Antenna factors were applied as appropriate, as well as cable loss, etc.  All readings were so close between units the differences were negligible.

Documentation on this very forum has shown FCC documents where these exact devices are used and referenced in filed complaints. 

The test site was a large open field.  The nearest building as at least half a mile away, the nearest trees were many hundreds of feet away.  The Whole House transmitter and the FIM-71 receiving antenna were at 7 feet above the ground as specified by Potomac.  It is worth noting that the receive antenna on the FIM-71 was horizontal for all tests (it's a half wave dipole adjusted in length for the frequency) as this is what it is calibrated for, I did however take readings with the transmitt antenna both vertical and horizontal.  You will see that, at least at 3 meters, antenna orientation has a huge effect on signal. I am posting all readings in uV.  You may convert to mV if you like. I was testing at 92.7 MHz, the frequency that showed a nearly imperceptable reading of background noise effect, somewhere around 1.5 uV/m (so technically you might subtract 1.5 uV/m from my numbers but it really won't matter. 

Here are the results, followed by some more comments.

At three meters, transmitter in "US power setting":

With Transmitter antenna vertical: 382 uV/m

With transmitter antenna horizontal: 1102 uV/m

At three meters with transmitter in "Canada power setting" (lightning bolt icon showing)

With transmitter antenna vertical: 4950 uV/m

With transmitter antenna horizontal 17775 uV/m

There are your basic tests at three meters between transmit and receive antennas.  I then did a quick range test using the Tecsun PL310 as long as I was in an open area. I didn't have the ambition to run a marathon to various points so I went out to 220 feet.  At US power at 220 feet signal was readable only by positioning the radio for the "hot spot" where the signal was strong and clear.  Now, this was directly broadside to the transmit antenna in a horizontal position.  As I walked around so I was at an end of the antenna (e.g. with the transmit antenna pointed at me) the signal was gone by about 100 feet.  So in horizontal position, as expected, the signal is quite directional.  The above tests were all done with just the transmitter on, running on new alkaline batteries, with no other cables connected to it.  But I noticed something a couple weeks ago while playing around with this transmitter out in the yard that I was curious to test out here in the open.

Apparently connecting any cable to the transmitter increases it's RF output. I'm guessing that connecting, say, an audio cable to the line in jack, the common (ground) of the audio cable acts as a counterpoise to the antenna, as there's quite an RF output jump just by simply plugging in an audio cable. Per haps it's one of the other leads in the cable acting as additional antenna, I don't know. And it also seemed to somewhat negate the effects of the horizontal or vertical antenna.  With the FIM-71 reading 382 uV/m with the transmitter in low (US) power and the antenna vertical, I plugged in a 4 foot audio cable -- 1/8 inch stereo to 1/8 inch stereo, and the output jumped to 4050 uV/m and this is just with the audio cable hanging down from the transmitter.  I was able to vary the output greatly just by changing the position of the audio cable in relation to the antenna.  I then tried it with the external power cable.  This is a short USB power cable provided with the transmitter.  Same effect, except not quite so dramatic of an increase. 

Then I though, what might the field strengh readings be at 10 meters? In high power, the secret "Canadian" setting, with the antenna horizontal it was 11475 uV/m.  Switching it to low "US" power the readings at 10 meters were 360 uV/m with the transmit antenna horizontal, and 112 uV/m with the transmit antenna vertical.  Just in case you're curious, with the provided long wire antenna (that has the warning not to use it in the USA) the field strength went to 17775 uV/m in the high power Canadian setting.  Although I wasn't really prepared to test this as I had no way to hold the antenna wire in the air so I just let it hang down from the transmitter.  Also as it was a breezy day the field strength varied as the antenna swayed in the breeze.  Had I been thinking I could have plugged the audio cable back in to see what I got!  Probably if the antenna were held up above the transmitter and the audio cable was straight down as a counterpoise I could really have had some power going!

My conclusion has to be that there's simply no way this is legal in the USA. Although, it's certified.  Makes me wonder exactly what the certification process involves.  Now, the Potomac FIM-71 indicates the margin of error for these tests at 7 feet above ground level is 18%.  So, in US mode, with the antenna vertical, and no audio or power cables plugged into the transmitter the reading is 382 uV/m. Allowing an 18% error in favor of the transmitter being legal (FCC documentation on this site shows the FCC always factors maximum margin of error in favor of the station) that still leaves us with 325 uV/m which is still above the limit of 250 uV/m. Then again, the margin of error could easily go the other way and be 439 uV/m, or somewhere inbetween. Whole different ball game if you flip the antenna to horizontal. Then it depends on if you're reading off the null ends of the antenna, or broadside. I didn't take any null end readings but I'm sure those readings would easily be legal.

Then, just for fun I took the transmitter up to my studio (second floor, old wood house, easily 20 feet above ground level) and set it in a window.  Then I set up the FIM in the backyard.  I was just sort of goofing around here so I didn't take any real readings, I just wanted to be able to see relative changes in field strength, but I was completely amazed at the HUGE difference in signal strength that could be induced sy simply plugging in the audio cable, and the differences between plugging it into an iPod compared to plugging it into a CD player (RF was quite a bit higher plugged into the iPod than a CD player). Or how simply moving the transmitter a foot to the left or right, moving the antenna to various positions horizontal, diagonal, vertical.  Setting my wire glasses next to the transmitter even made a difference. I tried probably 20 different things.  Moving the transmitter, moving the antenna, setting different things next to it, plugging it in to the ipod, then the CD player, moving the cable around, etc.  Every thing I changed changed the field strength.  Now, clearly I could have been simply been changing the direction of the signal, as well as varying the output by providing a counterpoise.  I finally quit because I had to run up two and a half flights of stairs from the studio out to the backyard and back and I got pooped. 

Trying to predict results, effectiveness, coverage, etc from this transmitter, and I suspect all Part 15 FM transmitters is a shot in the dark as a countless number of variables will effect virtually everything. 

So of course now I can think of a thousand different test combinations, but I'm done with this transmitter now.  So if anyone would like to buy a slightly used Whole House 3.0 at half price let me know :)

It will be interesting to see what the Decade CM-10 has in store for me. 

Tim in Bovey