I found the review I was looking for. It was about an LPB transmitter that some say was made by Hamilton.
I will be writing about my own experiences with the Rangemaster here soon.
WEAK-AM
I found the review I was looking for. It was about an LPB transmitter that some say was made by Hamilton.
I will be writing about my own experiences with the Rangemaster here soon.
WEAK-AM
Classical Music and More!
LPB AM transmitter
I assume that you are referring to the June 24, 2000 issue of Radio World that has an article about the LPB AM-2000. I was intrigued by your comment about this transmitter possibly being designed by Hamilton. I recently had the opportunity to ask somebody who knows about this. I talked to John Devecka, who was the VP of Sales and Marketing at LPB a few years ago. He now runs a college radio and TV station.
John’s father, Ed Devecka, bought LPB in 1988 from Dick Crompton, who founded the company in 1960. Ed sold LPB in 1998, but he still works there as an applications engineer.
John told me that Keith Hamilton was not in any way involved with the AM-2000. The designer was someone named Dale Head, who is now deceased. John thinks that the Rangemaster is superior to the AM-2000, paricularly because it has better modulation characteristics.
Very interesting!
Thanks Ermi, that was really informative. I’ve seen LPB products in the market and on eBay for quite some time. This was a really nice background on a well known product line.
Experimental broadcasting for a better tomorrow!
AM-2000 loading coil
Looking at the picture of the LPB AM-2000 in the Radio World article, on the right side of the box containing the milliammeter, there is a small loading coil wound on a ferrite rod. The picture, unfortunatrly, does not show the loading coil very clearly. When I asked John Devecka why LPB located the loading coil inside the box instead of using a larger, higher Q, air-wound coil on the outside, he cited compliance with the Part 15.219 regulations. He thought that the FCC may not approve of a loading coil outside of the box. The Rangemaster’s loading coil is the secondary of the RF transformer that couples the final stage of the transmitter to the antenna, which is wound on an iron powder toroid. The resemblance between the Rangemaster and the AM-2000 is striking. Both are boxes with CB antennas attached to the top. It’s no wonder that some people thought that both transmitters were designed by the same person.
There is a part15.us Forum topic initiated by Radio Joe about loading coils, to which WEAK-AM made a valuable contribution. He pointed out that Ben H. Tongue (co-founder of Blonder-Tongue Laboratories) has a website about crystal radio design, http://www.bentongue.com, in which he describes the construction of high-Q coils for crystal radios. WEAK-AM thought correctly that the information about good AM crystal radio coils is applicable to Part 15 AM loading coils.
Publication #29 on Ben Tongue’s website contains information directly related to making Part 15 AM loading coils using ferrite rods. The important points in that publication are:
1. The ferrite material should be NiZn instead of MnZn. The way to tell the difference is that NiZn gives no resistance indication with a DMM, but MnZn gives an indication.
2. The ferrire rod should be at least 3 times as long as the winding.
3. When using solid wire, small diameter wire gives higher Q than larger diameter if the winding length is the same. This is because greater wire spacing reduces the proximity effect.
The most disturbing caveat by Tongue is that he thinks that ferrite manufacturers are no longer bothering to anneal ferrite material. Annealing reduces core loss and therefore decreases Q. Annealing is a long process, and that is why it may possibly not be done. As Tongue says, “time is money.”
I had a problem some years ago with toroids made of Fair-Rite #68 material not being properly annealed. The toroids were for 1 uH coils for use at 30 MHz, and they had to have a Q of at least 300. Fair-Rite took the cores back and re-annealed them. Now the Q was 400, but this was not good. Annealing reduces permeability, and now the coils did not have enough inductance. My bargining power was limited because I only needed a few hundred toroids per year. The only way to solve the problem was to make it wothwhile for Fair-Rite to make another batch of ferrite material. I bought a lot more toroids than were needed for the life of the product.
A manufacturer of certified Part 15 AM transmitters might want to use an internal loading coil in order to avoid regulatory problems. The doubtful availability of good ferrites may make it necessary to intentionally make a transmitter that has low efficiency because of high loading coil loss. Tongue does not discuss iron powder materials on his web site.
Incidentally, when I talked to John Devecka, I mentioned that I was not able to get a schematic diagram for the AM-2000 from LPB. LPB provides schematics for several of their products on their web site, but not for the AM-2000. The service department wouldn’t help, either. John told me that there is no schematic for the AM-2000. Dale Head had his own private notes, but there never was any schematic in the company’s drawing files.
Internal toroid coil
First, I want to comment on the issue of using internal vs. external coils. When I built my first high performance Part 15 AM transmitter back in the ’80’s, I also wondered if the use of a large external loading coil would be permissible. Therefore, I designed the loading coil as part of the output stage matching network. It consisted of several very large Miniductor stock coils inside (and well spaced from) a large, rf-tight aluminum enclosure. The output ran Class E using a VMP-22 VMOS power FET, using a circuit published by Nathan Sokal. It was extremely efficient. I have circulated a few photos of it privately.
My latest exploits involve designing highly efficient toroidal loading coils, which could be mounted either internally or externally. I personally prefer internal mounting because it provides protection from the elements. My initial experiments using Type 2 iron powder cores have been very promising. I wound a 140 uH coil using #26 wire on a T-157-2 core. By contrast, I have not had very good results using ferrite cores so far. I should say that the main reason for wanting to investigate the use of ferrites is the higher permeability of these in comparison to iron powder. However, you can use larger iron powder cores to obtain larger inductance values efficiently. I just ordered some larger cores (184, 225, and 300) and litz wire to see what kind of performance I can get with that arrangement.
My personal preference is to employ top loading on the antenna to increase the antenna capacitance, reducing the inductance value required to match it. Then you can use a relatively modest loading coil.
There is another crystal set site that some of you might want to know about: http://www.1n34a.com. The owner sells litz wire and custom made variable capacitors. I plan on using his 410 pF variable cap to series tune my antenna. This requires the use of an inductor that is somewhat larger than the value that would be required for manual tuning, but somehow I really like the idea of being able to fine tune the antenna by turning a capacitor knob… smooth! The litz wire I decided on is the 330/46SPSN. It is kind of expensive, but should offer performance superior to solid wire.
WEAK-AM
Classical Music and More!