8W + PLL + Stereo + RDS

[Albert H]

Unfortunately, that type of filter is extremely rare. We used to use the Toko BLR-series filters in the NRG products, but they were getting hard to find by the turn of the century, and the ones that were available became very expensive.

To overcome the problem, I designed a gyrator-based elliptic filter for use in our stereo coders. There are two versions of the circuit: The original version used dual suppy rails for the op-amps. The second version was a re-design to allow single-rail operation, but had a slightly degraded specification when compared to the original version. That said, the single-rail version gave perfectly acceptable results.

[rigmo]

It is "FDP-02" filter from old polish Unitra Diora tuner (AS-252). Basically it's Toko 19kHz LPF equivalent.

26dB @ 19kHz and 50dB @ 38kHz.

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Is possible get more details as turns of coil etc to make new one?

[1608cc]

Is possible get more details as turns of coil etc to make new one?

Will be a bit hard to get that type of data. Internal schematic and capacitors values are published by unitra, but the inductance values are absent. Coils are wound on ferrite core and an external ferrite cap is applied on threaded enclosure to tune filters exacly at 19khz and 38khz. Also both inductors are splitted in the middle, so have to disassemble filter and desolder caps to measure each half of coil properly. I don't really want to damage these thin wires. I attached internal photos of FDP-01, have a pair of these - can check this type if You want.

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[rigmo]

Do you heave sample for sale or borrow i have possible offer to make copies..

[1608cc]

Remind me on the weekend via PM or email (same as my nick here + gmail.com). Can borrow You a pair desoldered from another tuner (Radmor T-5521) that I have on attic - we talk about FDP-02. Also interested in copies If you able to make them, FDP-02 are almost imposible to get as single part, only as you buy the whole device with them inside...

[Frequent Lee]

The old NRG Halfwave vertical worked well too. The extremely high feedpoint impedance of the radiator is transformed by the coils and capacitor matching section under the cowl. The problem with the aerial is that the capacitor would flashover in damp air when trying to run higher power throught the thing!

The 50 watt max handling version was usually good for 80 watts, upwards from that you could run 150 in winter or colder weather. The weak spot was the plastic tubing used as the former for the RF feed point coil after the tuning plates, they used to melt then sag from the RF heat and I often found the driven element pointed 90 degrees or more when I went to the rig site after noticing the signal meter needle on my tuner wasn't at it's usual 79% indication mark.

I used the same rig site and tried a single pawsey tuned dipole critically matched to the exact frequency and matched with my mfj meter, an Arreff style CB silver rod 5/8wave, Double stacked dipoles, J-Pole, Sandpiper 5/8 ring base and the NRG 1/2 wave famous funnel vertical.

The best real world coverage distance wise was the Sandpiper 5/8 but I preferred the coverage of the NRG for concentration of signal to my intended catchment area.

They were relatively expensive so I'd often do antenna rotation between the NRG and home made tuned dipoles using the method Stephen showed me how to make back in the early 90s.

I still have 4 150 watt NRG antennas in my loft, collectors items now.

[Albert H]

My favourite - for a cheap, quick, effective aerial was a ⅞-wave "Flowerpot". It gives useful gain over a dipole, is easy to mount, isn't obviously a pirate aerial, and gives an ideal radiation pattern - particularly from high sites. Two modifications I would usually make were:

1. To add a shortened groundplane (about ⅛-vave long) which reduced the residual downward radiation into the block

2. Add a small capacity "hat" made from stiff wire to the top, to somewhat widen the bandwidth, to make tuning less critical

The ones we made were housed in white PVC plumbing pipe, with nylon clamps to attach to the mast, with plastic end caps glued on to prevent water getting in. We'd paint them with acrylic paint to further conceal them. We used to be able to make them - including a length of feeder - for about £14 each!

[reverend]

If (later) you want to equalise the RF output right across the band, you could use a simple diode probe to measure the amplitude of the RF output, and then use an op-amp buffer to adjust the LM317!

It's also worth thinking about monitoring the current through the output stage. If you have a mismatch on the output of the transmitter and are measuring the RF amplitude at the output, it's easy for the output to drop and you try and whack up the gain to compensate. Simple example: if the output is short circuited, the RF detector will see nothing and as a result will try and drive the output stage harder. Meanwhile all that's happening is that the output stage is getting hotter and hotter. If you monitor (or even just limit) the output device current too, you can at least limit the heat/damage.

[Albert H]

Over the uears, I've used all kinds of RF output monitoring and automatic adjustment. The best sensor (in practice) is the standard Breune Bridge, comparing forward and reflected power. One PIC-based system I've used measured the reflected power, and would turn off the drive if the VSWR was worse than about 2:1. It also measured the forward power, and would adjust the drive level to give the pre-set output power.

One of my quite widely used PAs had a pair of MRF150s (this was in early FET PA days), and developed 240 Watts output. The drive power was adjusted to give 240 Watts, but only after the returned RF showed a good enough VSWR at half power (it would run forever into anything!). This proved to be a very reliable monitoring system, and would keep a station on the air (albeit with reduced power) if the antenna was damaged by bad weather.

Subsequently, a few years after I came up with this system, I came across a Bext PA for 10 kW that was a perfect imitation of my approach! Other manufacturers use similar designs these days, particularly because broadcasters demand high reliability, and PA devices have become so expensive!

Unfortunately, most pirates try to really push whatever devices they can get hold of, in an effort to get as much power as possible at the cheapest price. This is understandable, particularly as they expect their gear to have a very limited lifespan! Other, better, rig builders don't try to screw the very lasr Watt out of their components, They recognise that their rigs will be more reliable if they run cooler, and will be more tolerant of less than ideal aerials and feeders if they throttle the power back a bit.

Unfortunately, most of the people buying and using rigs are convinced that a 500 Watt rig will "get out better" than a 400 Watt rig, without understanding that they would need to DOUBLE the output power to get just 1 "S" point more signal at the receiver! These people simply don't understand that a well-located 50 Watt rig will outperform a poorly located 500 Watt rig. My puny little wind-and-solar-powered 40 Watt station in California was heard by tens of thousands of listeners, despite it competing in a market with >100kW stations! In that instance, height and good directional antennas made it work.

[reverend]

Unfortunately, most of the people buying and using rigs are convinced that a 500 Watt rig will "get out better" than a 400 Watt rig, without understanding that they would need to DOUBLE the output power to get just 1 "S" point more signal at the receiver!

I was chastised once by a self-proclaimed 'radio expert' for making that argument. In this case it was 80 vs 100 Watts. He told me that 20 Watts goes a long way and that if that much is missing from the signal it would have a big effect on the coverage. I can understand the logic in his argument, even if it is totally and utterly wrong!

[yellowbeard]

Even I know FM transmissions go to the horizon at all power levels, the enemy is noise in the receiver and the environment. I have done the experiment. Back in the day when there was not so much on FM we went from 50 Watts ERP to 1.3KW ERP, then did the exact same drive out to about 25 or 30 miles away. There was no real extra range but the hiss, intelligibility and fading improved every step of the way and the fringe had gone from "there's something there" to being able to hear the phone number. A couple of miles or so further up the road there was nothing. It was better everywhere but not further.