• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

working on an idea for a 2200 meter transmitter

Status
Not open for further replies.

unclejed613

Well-Known Member
Most Helpful Member
the FCC recently began allowing amateur operators in the US to use the 2200m and 630m (136kHz and 475kHz) bands. one of the hurdles for using these bands is that wire antennas are extremely inefficient. for 2200m, a typical wire antenna (a fan antenna or top loaded inverted L) takes about 250 or more watts to achieve the 5 watt ERP limit. magnetic loops can be much more efficient, because ground conductivity and other environmental factors don't reduce the efficiency. the problem with mag loops, however is their small RF "aperture". but i think i saw the solution to the aperture problem when i was in the army. this was in the form of an array of loop antennas strung out about 5ft apart across a field. the whole array was about 30 or 40 ft. it was a nice "stealth" antenna as well because the loops were about 18" diameter. these loops were being used in the HF band, and i actually heard that station on a communications receiver several times on frequencies from 2Mhz to 15Mhz. a mag loop for 2200M is going to be a bit bigger, but the real challenge is making an inexpensive high voltage tuning cap. most transmitting mag loops use vacuum variables, which can get pricey. using a standard air variable can be a problem because the air gap voltage is usually only several hundred volts. so i'm looking at some "out of the box" ideas, like making a variable cap from pieces of single-sided PCB, or concentric pieces of PVC pipe with copper foil wrapped around the PVC pipes. i've got a lot of books from the 1910-1920 era, like "Practical Wireless Telegraphy" and there are some very interesting cap designs (such as a cap that operates like opening and closing a book). a lot of this type of technology is used in constructing Tesla coils ( FCC rules say spark-gap transmitters, damped wave, are type B emissions, and are strictly verboten).

so the main hurdle is the antenna array. another "out of the box" idea i had is to use a Howland Current Source to drive the antennas, rather than a typical single ended RF amp.

i would like to see if anybody has any interesting ideas for the capacitor. the tuning cap needs to be in the neighborhood of 2nF, and i'm not sure yet what the voltage will be, maybe 10kV. the cap needs to be able to handle relatively high antenna current ~10A or so. it will also need to be enclosed in a weatherproof enclosure.
 

RadioRon

Well-Known Member
Have you considered using a straight wire antenna that is actively driven at each end, or in other words a differential drive setup? I know this doesn't really solve the radiator efficiency problem, but would it not make the drive impedance problem a lot easier? Using two amplifiers, mounted at each end of the wire to eliminate the need for transmission lines between power amp and radiator would allow you to design the amps with very low output impedance, which may help with efficiency.
I have no idea if this would help eliminate the need for a high voltage cap, but if everything is low impedance and high current, perhaps voltages will be relatively low.
 

DerStrom8

Super Moderator
Most Helpful Member
Back when I was building SGTCs I tossed around the idea in my head of building a variable high voltage capacitor that involved using two metal plates, one fixed and one threaded (with insulation) on to a threaded rod, with a soft dielectric material between them. As the rod is turned (by some sort of hand or motor crank) the top plate can be moved closer to or further away from the bottom plate, squishing the soft dielectric between it. As the material becomes flatter the capacitance increases, and as pressure is released it increases. I never got around to prototyping this setup but I wonder if it might work in your application. It would be possible to stack multiples of these in parallel on the same threaded rod if higher capacitance is needed.

Let me know if you'd like me to put together a sketch to better illustrate the idea. Words do not describe it well.
 

dr pepper

Well-Known Member
Most Helpful Member
Not sure if I got that right, can you overtune with a bigger cap and then null some of the pf's out with a inductor rather than a cap, a rollercoaster inductor is probably going to be easier to make or purchase than a high power trimmer cap.
I'm interested in the lowfer band too, its unrestricted (apart from erp) here in the Uk.
 

JimB

Super Moderator
Most Helpful Member

dr pepper

Well-Known Member
Most Helpful Member
Cant be bothered to find out as I dont use the band.
 
Status
Not open for further replies.

Latest threads

EE World Online Articles

Loading
Top