Mike's 1420 MHz, Hydrogen Line, Radio Telescope Project Page
Project -IN PROGRESS-
Phase -Concept & Design-
Last updated 12/18/08
BEST VIEWED W/ MS-IE at 1600 x 1200 RESOLUTION !
I decided to write up this page to help other folks in the planning, installation, and debugging of a 1420 MHz, Hydrogen Line, Radio Telescope, for amateur radio astronomy (RA) use. This project was conceived on 12/2008 during a conversation between Pat, W3PO, Tim, N3VNG, and myself on a 222 MHz repeater.
We are all Interested in RA, Ham operators, and junk collectors. We have on hand a TON of old equipment, antennas, dishes, and TVRO equipment. The goals of the current project are as follows:
To use as many "on hand" construction parts as possible,
To reuse as much of our existing ham stations as we can,
To keep the cost as low as possible,
To keep it small, and simple,
To make it as fun, and interesting as possible!
So, with the intention of passing on what we're doing and learning.... Here we go!
1420 MHz Hydrogen Line RA - Theory & Practice:
What's this project all about? What can we do with this Radio Telescope, and why would we want to? All that will be explained first.
I'm Not going to re-write the book here, but rather republish a lot of excellent information from other sources available on the net. With credit, of course, to the original authors & sites.
Continued on a separate page see link below
CLICK HERE FOR ALL THE THEORY, CHARTS and GRAPHS YOU'LL EVER WANT!
Concept & Design:
Project Plan, Parts List, Getting together what's needed from where......
Much of the project plan is dictated by the goals that were set forth in the overview. The first consideration was the antenna. Something small, easy to construct, with parts on hand, and still with enough gain to be useful for the project. The conclusion was a 15 turn helical antenna centered at the 1420 MHz target frequency. Once the antenna was selected, the focus turned to what receive equipment to use. Initially we were thinking of using a communications receiver such as an SDR, Icom or AOR radio that already covers up to 3 GHz. However, none of us own one, they are pretty pricey, and you still have the issue of getting a 1.4 GHz signal from the antenna to the rcvr without loosing all your precious weak signal in between!
Thus, it occurred to us why not do what the Television Receive Only (TVRO) people do, and build a Low Noise Amp (LNA) at the antenna itself, and couple it immediately to a down-converter bringing 1420 MHz down to a manageable intermediate frequency (IF) that we can use our existing ham equipment to receive! (OK, you pros out their are probably going, "duh! what else would you do!", and "Why are these guys reinventing the wheel!". Well, you have to remember we're not pros at this, and a lot of the "fun" is learning things [goal 5]. There is a bit of fun in coming up with this autonomously and then discovering that you've fallen onto the exact same approach that is considered SOP! i.e., your thought process must be pretty well, "on track!".) Anyway, I digress.
So now we have a plan. Helical antenna, LNB/Down-converter (we'll call this an LNBF sticking to the TVRO metaphor) and a ham receiver at some HF/VHF/UHF IF frequency as our RA receiver. Later we'll start thinking about a computer interface (i/f) for spectrum analysis, and data gathering and recording.
There are a lot of possible choices for an antenna. The best choice is probably a dish. Two of the 3 of us actually have old C band TVRO dishes in storage, but you have the issue of size, placement, neighbors, and coming up with an Azimuth-Elevation drive for it. TVRO dishes come with mounts and drives that simply scan across the "Clarke Belt" where the satellites live. If you want to scan the WHOLE sky, you need to have an Az-El drive that will let you point it at any spot in the sky, and let you track the object as the earth (and/or the object) moves. Much the same as one needs to do with the amateur satellites. Building or buying an az-el drive for a 4 meter TVRO dish gets costly violating a couple of our goals. You could also build Yagis or Log-Periodics, but at the frequency we're talking why not get the most gain for the buck, keep it relatively small, and use parts most everyone already has laying in their garage! We chose to build a helical antenna.
The only parts you need to build a helical are a piece of PVC pipe, some solid copper wire, a piece of scrap metal or screen, and a coax connector. All in the correct quantities, and dimensions, of course.
Your Basic Helical Looks like This:
The dimensions referenced can be calculated from:
The pitch angle used is nominally 12 degrees for the above calculations and diagram. The center "insulator" or support for the helically wrapped wire should be an insulator of the "smallest possible" dielectric constant! The wire diameter "d" is not critical #12 to #18 AWG solid copper is fine.
OR they can be calculated from this nifty on-line calculator tool: http://www.csgnetwork.com/antennahtcalc.html
Thus, for our 1420 MHz Helical the calculator gives the following dimensions:
Required Data Entry
|Number of Turns||Turns (4-36)|
|Wavelength Reflector||Inches cm|
|Spacing Between Turns||Inches cm|
|Length of Each Turn||Inches cm|
|Length of Wire Needed||Inches cm|
|Antenna Length||Inches cm|
In other words, a 3" diameter ("D" from above) 31" length of PVC pipe can be used as the support. A 9" ("R" from above) circular cut piece of scrap metal, or tight weave screen can be used for the reflector. And 140" of solid copper wire wrapped onto the support, with a turns spacing of 2" ("P" from above) can be used as the element.
Why 15 Turns? If you play with the calculator you'll find that the next amount of turns that will get you any significant increase in gain is about 36 (for +3dB) which increases the length to 79". This now violates goal 4, "Keep it small".
The LNA/Down-Converter (LNBF):
The first thing we looked at was a commercially available LNA made by Down East Microwave Inc. (DEMI). It's made specifically for RA work, and 1420MHz. With > 16dB gain and a BW of 1.4 to 1.5 GHz. (you can find it here: http://www.downeastmicrowave.com/PDF/Ulna.PDF ) It's cost is about $140.00 if I remember the catalog correctly. The next step was to come up with the down-converter. During this exercise I made an interesting discovery. RA folks don't like to publish designs in the public domain! I found lots of references to 1420 MHz radio telescopes, many referred to the "down-converter" none gave a schematic. Many referred to a particular person, or design, but a search for it usually just led to a product that had to be purchased (usually discontinued!), that was generally well over $500.00! So, after searching about and only hitting dead ends, and pricey products, I had another obvious epiphany. Why not use the plans for a 1296 MHz Ham transverter and just change the Local Oscillator frequency (LO) to the mixer to make it work for 1420 MHz !?!? (yeah, another duh! moment)
That's when I came across the Excellent transverter plans created by Mike Brink, ZS6MEG in South Africa!
Mike's design is actually quite perfect for RA purposes. Here's why:
It has a built in, dual stage, broadband (DC to 6 GHz), LNA as part of the design! Making it quite versatile, with an estimated 40dB of gain at 1296 MHz
It has a PIC controlled synthesizer and VCO that is very broadband. Generating an LO from 1050 to 2100 MHz. So if you use 435 MHz as the IF you can tune from 615 to 1665 MHz!
The only modification that needs to be made is in the front end band pass filter portion of the LNA which is currently calculated at a 1300 MHz center. It only needs to be shifted to 1420 by changing the LC values!
The units can be mated with TVRO LNBs to allow for RA work in the 5-6 GHz and 10-12 GHz range as well!
The PDF plans for the transverter are here: http://www.zs6meg.co.za/Transverter.pdf
I need to print them out and comb over them a bit more, but it would seem that with just the Synth, VCO, and Filter Modified Down-Converter, we're in business!
This is now the simple part. Any 70cm, all mode ham rcvr can be used. For RA purposes you'll need SSB capable receive. I plan on using my Icom R7100 all-mode receiver as my "base unit".
There are a few other items that we will need to think about to complete the project. As for now, I've not given them a whole lot of deep thought, but they will need to be addressed:
An Az-El rotor for the Helical and a tracking system. For my part I have a Yaesu Az-El rotor, I just need some RA software to interface to the controller to be able to point and track. We'll need to come up with something for the other guys.
Some RA software for the shack computer and a sound card interface for the rcvr. Will need that to get useful spectrograms and data from the radio telescope.
This is where we stand so far in the project. We're getting close to the end of the Concept and Design Phase!
Not Yet Begun! Wait for Construction Phase
Testing & Debugging:
Not Yet Begun! Wait for Test Phase
More will be forthcoming. Stay tuned.
I hope this has helped SOMEONE in SOME WAY either as an intellectual exercise, or a practical one in installation.
Best to all, happy radio days, and good DX.....
M. K. Hess 12/18/2008
Mike Hess - N3EAQ - N3EAQ@N3EAQ.NET
Links of Interest:
Down East Microwave: http://www.downeastmicrowave.com
Comments, and Feedback: