One method proposed to control the field strength and thus the range for part 15 FM is to adjust the antenna length. I did a mini study using my FM transmitter to find out how much control changing the antenna length provides.
My Ramsey FM-25A is enclosed in a metal box with the supplied telescoping antenna protruding through a hole in the top of the cabinet. To gauge the field strength I tapped into the AGC line of a EICO HFT-90 tuner (yep, vacuum tubes) and connected the 300 ohm differential antenna input to the output of a Motorola test set. The test set output was varied and plotted against the AGC voltage and, as expected, the relationship was not linear. A 1 foot long antenna was attached to the receiver and the other antenna terminal was grounded to the receiver chassis. This arrangement gives a relative measurement of the field strength by use of the graph to find the receiver terminal voltage corresponding to the AGC voltage reading. The receiver was positioned 3 meters from the transmitter.
The data were recorded for several lengths of the transmit antenna, corrected with the graph for the non-linearity, and normalized to be 100 at the maximum antenna length.
Length (inches) Rel. FS
17…………………..100
14…………………….85
12…………………….81
10…………………….77
_8…………………….62
_6…………………….15
_4…………………….08
This shows that for my particular transmitter the field strength drops rapidly as the antenna length is shortened below 8 inches. For lengths above 8 inches the relationship is approximately linear and deviates from this for shorter lengths. The curve for the AGC vs. antenna voltage has a small slope for voltages below those produced by the 8 inch length hence there is probably significant measurement error for these settings. Time permitting I may lengthen the receiver antenna to get a higher voltage on the AGC and repeat this.
Neil
Just curious
Neil,
Just out of curiosity did you have to adjust the transmitter when making changes to the length of the antenna? I am not familiar with the circuitry of the Ramsey FM 25 A , the FM 25 B has a coil or pot ( L1 ) for adjusting the voltage. I noted on the FM 25B that the voltage changes if you change frequencies or use a different antenna connected to the F-connector.
When i was using my ground plane i heard distortion in the audio and cleared that up by adjusting L1. Now that the FM 25B is back to using the whip again, i had distortion again until i made some adjustments to L1. I just wonder if that would change your experiments or have no affect at all?
I like this experiment your doing and look forward to hearing more, it gives me a good indicator as to what i could expect out of my transmitter if changes are made to the antenna.
Reply
Barry,
No adjustment was made when I changed the antenna length. The FM-25A is the older model in the series and it uses a bipolar transistor as the output stage. There is no circuit adjustment for the output power. It survives with no antenna all the way to a short circuited antenna (whoops). The thinking regarding the antenna length affecting the radiated field is that as the antenna is shortened the transmitter load becomes highly reactive and less power is transferred to the antenna. The radiation resistance would also be lowered further reducing the efficiency. I have not tried using a 1/4 wave antenna but may do so just to find out what happens.
I and my two sons each have one of these and the effect you noted about changing frequency and trimming L1 has been observed. Contrary to the instructions I trim L1 whenever I change frequency and tune it for best sound which seems to go along with the strongest signal. This is just my impression since I didn’t actually measure the output strength.
There is a better technique for measuring the signal strength than the one I used but it requires an accurate RF attenuator and I am not sure I can build one which works properly at 90 MHz but I am pondering doing so.
Neil
So this distortion
So this distortion is possibly caused by the oscillator not receiving the proper voltage. Since your transmitter is the 25a and mine is the 25b there seems to be some similarities in the circuits even though they were probably designed years apart, the circuits may have the same theory for operation.
I will have to dig around and see if i have the schematics for the FM 25a, I already have the schematics for the FM 25b on my desk. It would be interesting to see the circuits side-by-side to compare.
Back to the subject at hand.
The idea to reduce the coverage by shortening the antenna length is by far the best way to limit our stations field strength.
We can limit or increase our coverage according to our antenna’s efficiency or inefficiency. The Scosche FMT -4 , Maxell FMT-1, Maxell P-13 all use a short or no antenna at all to achieve the part15 limits and compliance with the FCC. I have a few questions, is it possible to use attenuation to reduce rf power before it reaches the antenna that is cut for use on our frequency ? What would happen if i reduce the rf power on the 25B and then add attenuation until i reach the legal field strength?
I know Ramsey Kit Transmitters have no trouble operating non-stop with a less than ideal antenna. In fact i have been guilty of forgetting to connect the antenna and wondered why my range was so bad.
In my mind, a mismatch spells disaster for most transmitters, swr is usually impossible to adjust for hobbyist since most of us don’t own a swr meter made for the fm band. The harmonics introduced into the circuit can add more fuel to the fire. So what makes these transmitters so robust?
Well , im just full of questions tonight aren’t I?
Reducing FM Transmitter Output
Barry,
My thoughts regarding your questions:
I have a few questions, is it possible to use attenuation to reduce rf power before it reaches the antenna that is cut for use on our frequency ? What would happen if i reduce the rf power on the 25B and then add attenuation until i reach the legal field strength?
The output power and field strength will get smaller. One of the site sponsors sells attenuators. Click their link on the left. An advantage to attenuators is they can provide a rather constant load impedance to the transmitter which makes it less susceptible to changing antenna conditions. One problem with attenuators on transmitter outputs which is often overlooked is proper shielding. Attenuators won’t control field strength as calculated if some of the higher power signal is leaking. The short antenna works for me and it is simple.
In my mind, a mismatch spells disaster for most transmitters, …[snip]… So what makes these transmitters so robust?
The secret is the low power at which the output stage operates. At milliwatt levels the transistors are essentially loafing. Usually no SWR protection is needed for powers up to about 2 or 3 watts. Above this, especially for handhelds, some control is needed such as a directional coupler with its signal used to control the power gain of the transmitter circuit. My 25 watt 2 meter rig has this and it has prevented damage during those oops situations.
…swr is usually impossible to adjust for hobbyist since most of us don’t own a swr meter made for the fm band.
It is not critical to have a perfect SWR at the power levels of transmitters such as the Ramsey 25 series since transmitter damage is not a concern. My experience has been (on the 2 meter band) that if an antenna is cut to the proper calculated length and the correct coax is used the SWR will be below 1.5:1. The power loss due to this mismatch is small. (Edited to add that I found a table which illustrates the effect of SWR here: http://www.firestik.com/Tech_Docs/SWRLOSS.htm )
I don’t know how the 25A and 25B compare but I suspect the frequency synthesizer and PLL are the same. If you find your other schematic it would be interesting to compare the two.
Neil
I found one
Well my record keeping is terrible.
I was able to find schematics for the Ramsey FM 25a but i just can’t find the schematics for the Ramsey FM 25 B. I’ll continue to look further.
We have a flatbed scanner here and i was going to scan a pic of the schematics for the 25b but the scanner has other plans. The silly thing scanned a portion of the image and it wasn’t very clear. We have had this scanner a long time so it may have given up the ghost just sitting around idle.
So here are the schematics for the Ramsey 25 A :
http://radiodizzy.webs.com/SCHEM/ShemRamsey25a.jpg
I will post the other ones as soon as I find them.
Interesting
Hi Neil: This is interesting. If you come up
with anything else let us know.
I may have input at some point.
Best Wishes
Bruce, MICRO1690/1700
I agree
With my C Crane attaching a larger antenna makes the signal a whole lot better. Because You are making the Antenna the proper wavelength for the frequency. Mine at the moment is between a 1/4 length and 1/2 length of 99.9 which gives me pretty good range for a part 15 FM. (total reach of about 800 feet or so)
Better Method For FS Measurement
In my reply to Barry above I commented about a better way to perform my experiment and due to lack of time to pursue this I think it appropriate to describe the method in case someone wants to pursue it.
The problem with my experiment is dealing with the receiver AGC non linearity and I did so by using graph techniques. The difficulty is that at low signal levels the AGC voltage changes very little with the voltage at the antenna (small slope comment in my other post). This can lead to large errors at low voltages.
A way to deal with instrument non linearity is to use a calibrated attenuator to adjust the signal into the instrument so it indicates the same reading for each measurement. If the correspondence between this reading and what is being measured is known then it becomes a matter of just reading the attenuation and calculating the actual input signal from this.
What I suggest is using a potentiometer on the input of the receiver but getting one to work accurately at 90 MHz, proper matching to the receiver antenna terminals, and verifying its performance can be a challenge to the hobbyist. Nonetheless, maybe someone with the skill and a good lab can make this work.
Neil
Two Subjects
I compared the diagram provided by rock95seven to see differences between 25A & 25B, and they are entirely different using different IC chips.
That’s the first subject of this note, now the second.
Last Wednesday I put in the order for the new Whole House FM 2.0 and today Monday it already arrived by priority mail, and I have it set up in a window so it will reach in and out the doors, and here is a first report.
The stub antenna wire installed is 3.5-inches, which is of course even shorter than the 11-inch telescope on the C.Crane, and so far I’m testing this “as is” mode, finding reception best at very close range, except that the FM Sangeon ATS505 way at the other end of the house picks it up perfectly at 107.1.
130″ of extra wire and a quik connector are provided with a caution that U.S. listeners should not use it until they go to a country where more power is allowed.
Coming soon I will test it with a 1/4-wave antenna and write down the result for inclusion in my 10,000 pages of part 15 related papers.
Re the Whole House “extra wire”
It will be interesting to read your report of the effect of an extended antenna though the outcome should be no surprise. The 130″ extra wire is a rather odd length. Usually the best gain advantage with a simple antenna is from one of 1/4 or 5/8 wavelength.
It is also interesting since a certified transmitter is required to provide an antenna and if a connector is used it needs to be a non standard type so the user cannot attach another antenna. But that is another topic so I’ll stop here.
Neil
Continued Testing
Now with a 1/4-wavelength vertical, the added length attached via a clip on connector, not the one supplied, which did not allow the wire full access to the entry holes, but a size larger which I had.
The length is 24.070027-inches, using a formula supplied by Whole House.
11811 divided by frequency equals wavelength in inches.
The antenna is taped to the surface of a rear window on the theory that it will send a signal out through the glass in one direction, back into the house in the other. In previous threads I think we discussed glass as being fairly transparent at FM frequencies. However, I believe the antenna is acting as if it were taped against the wall, and is not doing much better than it did with the 3.5″ antenna.
On some radios I am getting a full scale signal according to power meters, but a lot of scramble noise. The transmitter is set on mono.
I think perhaps the transmit antenna is loaded with nearby station signals which are not so far from here at 100kW each.
Finding a location for an indoor transmitter usually means near a wall or in a corner or lower than desirable, unless we build a tower structure or ceiling hook.
The next time I test the Whole House FM will be in a different, we hope better, location.
The Crackle
Closer study has revealed that the crackle detected from the Whole House 2.0 is not coming from either the antenna or the transmitter, it was coming in on the audio line. Disconnecting the audio brought a clean quiet carrier.
I’ll describe the audio chain I used, which goes to explain how odd sounds are possible. The Whole House was pulling audio from the output of a Techniques Stereo tuner tuned to 101.9, receiving the link coming from the computer over the C.Crane Transmitter. The Whole House was also sharing a splitter to the SStran at 1550 AM.
One other thing important to mention is the LCD (liquid crystal display) panel, which is not as crisp and clear as it appears in the ad. Well, it looks JUST like the ad, until one removes the film decal that protects the face of the LCD. Everything shows up very softly on the blue panel, and must be viewed from an angle from the bottom of the unit. My best reading glasses sort of bring it into view. I think you, Bruce at MICRO1700 would be very frustrated by it. But maybe you have better magnification than simple glasses.
Another odd experience happened when I tuned in increments across the dial of the C.Crane C.C.Radio Plus and noticed the power levels hung always half way on the scale even with no station, maybe that’s digital FM, but I heard telemetry beeps and computer like bursts all across the dial. Maybe from my computer? But maybe from 2 blocks away the regional communications tower with cell phones and the entire emergency system for the area. We’re probably being micro waved to a good fry.
This has been round one of testing the Whole House FM 2.0. Onward now to find a different setup for more tests.
Whole House 2.0 and Other Field Strength Stuff
Hi Carl! Thanks for your comments on the
Whole House 2.0. You are right, I might
have problems with the display, but somebody
else could set it up for me. I am pleased that
the transmitter does mono.
I have been conducting a very informal study
on Part 15 FM field strength (about 48 dBu at
3 meters(?) Using Radio Locator.Com, I have
been looking at the coverage areas of FM stations
that are semi-locals, and their strength contours,
that are indicated as 40, 50, and 60 dBu. This
is sort of giving me an idea of what
48 dBu @ 3 meters
for Part 15 should
look like on a receiver signal strength meter as
compared to these other semi-local stations, using
the same receiver and receiving antenna. This is
not an exact science, it’s just an ongoing collection
of data to look at.
Best Wishes
Bruce, MICRO1690/1700
Whole House 2.0 Review Addendum
This report will be an addition to the previous report and is not the entirely different Second Review that is planned for later.
So far the review was hampered by crackly audio, which I have explained. So I moved the transmitter to a different window where there is a direct audio wire link. I taped the 1/4-wavelength antenna vertically along the edge of the window so the signal would theoretically go out and in.
25-feet away the CC Radio Plus picked up a full scale reading at 107.1 and the audio was crisp, clean, robust. That’s what we hoped to find. Very good audio, same as C.Crane transmitter or any pro-quality FM transmitter.
Very poor reception on the Grundig FR-200 walk-around portable, but this radio has a raggy FM tuner, only receptive to the full powers. But it is good for AM sensitivity.
On the next test I will set up the Wholehouse 2.0 with conditions exactly equal to the C.Crane xmtr and report on their comparisons.