SStran AMT3000 - Adding an Aerial Tuning Indicator

[igranic]

The AMT3000 is an excellent AM modulator (I use the word advisedly) for testing old radios, and is tunable over the entire AM band (and some more). However, the aerial does require tuning in order to obtain optimum output. As supplied, this requires the cover to be removed to allow a DMM to be connected to the test points and to adjust the aerial trimmer capacitor for maximum voltage. To make it easier, I have made some minor modifications to incorporate a tuning indicator and provide a means of access to the aerial trimmer as illustrated below. The tuning indicator also provides some visual reassurance that the AMT3000 is operating correctly. Note the additional hole at the rear right of the top cover.



There is sufficient space, at the right hand side of the control panel and above the logo, to pierce a rectangular hole to accommodate a small small edgewise meter of the type which was used in portable cassette recorders and some radios. Having several of these to hand, I decided to try out the idea.

As the meter is simply required to indicate peak output, the scale does not need to be calibrated. These meter movements generally have a FSD (full scale deflection) of approximately 200uA, so, by some straightforward measurements and simple arithmetic I arrived at series resistor value of 150k ohms.

Having cut the hole in the front panel using a small drill bit, a piercing saw, and a file, the meter unit is fixed to the inside of the front panel with two blobs of hot glue. The negative end of the meter connects to an earth point adjacent to T3. The positive end, via the 150k, resistor connects to the T1 end of R15. Jumper S7 is inserted in order to bypass R14.



A quick test showed it to be working, and I was able to trim the aerial to maximum output using the new meter.

The next job was to provide easy access to the aerial trimmer capacitor by drilling a suitable hole in the top cover exactly in line with the the centre of the trimmer. With the top cover removed, measure the distance from the centre of the trimmer to both the rear and side edges of the casing. Use these measurements to accurately mark the centre of the required hole on the underside of the top cover, then drill the hole.



Dropping a trimming tool vertically through the hole, it should be fairly easy to locate the slot by feel. For those who find this difficult, a suitable guide bush can be added by gluing an old plastic coil former or similar flanged plastic tube to the underside of the cover and cutting it so as to just clear the trimmer capacitor when the cover is fitted.

Edward

[paulsherwin]

I'm sure Edward would agree with the following comments.

People should beware of tweaking the output of a microtransmitter for a particular aerial, as this can *dramatically* increase the effective output. In some circumstances this may be perfectly acceptable and indeed desirable, but it's a good idea to check the transmission range after doing this to ensure that no problems are being created for neighbours (and no unwelcome regulatory attention is being attracted).

I've no personal experience of tweaking an MMT3000 in this way, but I have tweaked my Wild Planet Radio DJ transmitter (and rapidly untweaked it when I realized how much the range had increased).

Paul

[igranic]

In the circumstance where a reasonably efficient aerial (i.e. long wire, or large base-loaded whip) is being used, I agree with Paul about the need to strictly limit output. However, for indoor use a two or three metre length of wire is usually sufficient, which in my experience barely carries outside the property boundary.

However, for best enjoyment, it is good practice when setting up the AMT3000 to select a frequency where it and the adjacent channels are not in use, thereby avoiding any incoming or outgoing interference.

However, as the majority of vintage UK radio dials are calibrated in metres rather than KHz, and being fundamentally lazy when it comes to mental arithmetic, I have used Excel to create a modified version of the switch settings table (shown on pages 30, 31, and 32 of the manual), to show the settings in metres (rounded to the nearest integer) as well as kHz.

N.B. this is only for 9kHz channel-spacing versions of the AMT3000.

AMT3000_S4_Switch_Settings_9kHz.pdf

For the hexadecimal/binary numerate, I have also added a column showing the setting, with Bit 8 being the most significant bit.

Edward

[paulsherwin]

Originally Posted by igranic

In the circumstance where a reasonably efficient aerial (i.e. long wire, or large base-loaded whip) is being used, I agree with Paul about the need to strictly limit output. However, for indoor use a two or three metre length of wire is usually sufficient, which in my experience barely carries outside the property boundary.

You might be surprised about this. My Radio DJ is an American microtransmitter which is FCC approved and sold in the US as a toy. It is powered by 4 AA batteries. After adjusting the aerial trimmer for the supplied 2m trailing wire aerial, I found the signal was easily receivable around 100m away. As originally configured the range was about 15m.

Paul

[Steve_P]

Quite possible, Paul. When going about Radio Transmission, legal or otherwise, the power in the amp is only a part of the issue. Aerials come in all different shapes and sizes and the real power is here.

With the right antenna, 100mW can work a heck of a long way. An AM RSL will be about a watt in some cases and it can cover a town if the right aerial is used.

On the SSTRAN site, there are some interesting ideas, and I am not providing a link....

Cheers,

Steve P.

[Nickthedentist]

Nice work, Edward, thank-you for this.

Nick.

[radiogammon]

Thank's for that, Edward, I will give it a try.

[Hybrid tellies]

An interesting idea. With mine I fitted it into the workshop with a few meters of cable as an aerial and a mains earth carefully chose a clear frequency and peaked it right up and have been delighted with its performance. I recently did a range check it covers the house (3 bed semi) nicely and just makes it out to the end of the drive. Its only on more sensitive radios that it can be picked up slightly further out. This is ideal as it won't attract the wrong sort of attention.

[igranic]

Following further tests with this modification installed and with jumper S7 in place, I find that there is significant distortion at the high (frequency) end of MW, presumably because of asymmetric loading via C16 and D6. With S7 removed the distortion disappears, but there is then inadequate current to drive the meter.

I have overcome this by the addition of one additional component, an emitter-follower transistor driving the meter through a 27kohm resistor, as shown on the attached portion of schematic.