Millimeter-Wave propagation is quite interesting, compared to the other microwave bands. Mother nature has created a good area for those who like a challenge on the bands 24 GHz & above. Unlike the lower bands, there is quite a lot of excess loss above 20 GHz which is caused by molecular resonances with water (H20), and oxygen (O2). On 24 GHz, we have primarily water vapor loss to contend with, which adds a substantial amount of attenuation to the free space path loss. On 47 GHz, we have both water vapor loss, and oxygen loss to challenge our weak signals on their journey to the other-guy's receiver. Oxygen loss is fairly constant at approximately 0.16 dB/Km at sea level on 47 GHz. If you can go up to a 15,000 foot altitude, you will start to observe less oxygen loss, but most of us don't have this option. (In space, there is no O2 or water vapor, so the path losses would be nice and low...but you couldn't breathe, and your equipment would probably overheat without large heatsinks installed....AND...there are NO grid squares up there...yet...hi....imagine a 3D rover path in space...yipes!)
Here's a graph showing some comparisons of good 24GHz propagation conditions with bad ones. 24GHz Path Loss Graph
Note that a very well-equipped station on 24 GHz, working a similarly equipped station has a path loss tolerance of about 244dB!! (19in dishes, 2dB NF, 100mW). There's lots of potential for DX, waiting to be discovered.
As you can see...summertime is pretty difficult on 24 GHz, but then it is very interesting to learn just what can be worked in high temperature/humidity conditions. For the real DX, go out on a cold dry day in winter.
Here's an interesting graph showing the required SNR on 10 GHz to have a threshold level signal (just workable on CW) on 24 GHz. 10GHz/24GHz Comparison. Notice the extreme values for long-haul QSOs at 90degF. To work a station on 24 GHz at R=200km, on a 90degF day with only 50% relative humidity, he would have to be about 70dB out of the noise on 10 GHz!! You would need a good LOS path, and good equipment to attempt such a QSO. Be aware the data in this graph assumes equivalent antenna gain (you probably have more on 24G, right?), transmit power, and receive noise figure. The data is basically just the path loss difference, taking the water vapor absorption into account. You will probably need a few more dB that the values shown, if your 24GHz gear is like most of us (much lower TX power than 10GHz). This graph really shows you that DX on 24GHz is a cold weather game (unless you're in Riyadh where the humidity is usually low). It's still very interesting to go out and gather some first-hand experience with 24GHz propagation in hot-humid weather. It's surprising how well you can actually do on good paths.
I calculated the atmospheric losses for 10GHz, but they were less than 0.1dB, even at R=400Km (so I left this out of the data). Water vapor and oxygen just don't hurt 10GHz very much. 10GHz would be a very good liaison band for mm-wave QSO attempts. (I note that UKW Bericht sells a 10/24GHz dual-band feed...there's also a really cool 3-band feed described for 10/24/47GHz at http://www.dl6nci.de).
Here's another graph showing the relative performance of the 24, 47, & 76 GHz bands at 50% relative humidity. Atmospheric Loss 50% Relative Humidity
Here's another graph calculated for 80% relative humidity. Atmospheric Loss 80% Relative Humidity
Notice that these graphs include water vapor losses, and oxygen absorbtion. On 24 GHz, oxygen loss is minimal. On 47 GHz, it averages about 0.17dB/Km, at 68 deg F. Here is a graph of Path Loss vs Temperature (excluding 4/3 earth curvature and obstacles) for several different RH values. 47GHz Path Loss.
Even a high-performance station on 47 GHz (11dB NF, 30mW Pout, 38 dBi dish) can only handle a total path loss of about 233dB, which is limited to a maximum working distance of about 283 Km on an average day (40% humidity, 60 deg F). The real problem with 47 GHz dx is that this loss doesn't even account for the earth curvature, so you would need to be up on 6000 ft mountains on both ends of the path to get RLOS (radio line-of-sight)! If you find a pair of sites like this on the east coast, please let me know! The Europeans have a great advantage here with the Alps and the Pyrenees mountain ranges. Maybe now you can see what makes the 47 GHz band fun and challenging. I like messing around in the mountains...especially with good-working microwave gear.
I haven't ventured into 76 GHz as yet, but we're working on it. Here, the working range is even more challenging than the lower MM-wave bands. It is also much more difficult to produce more than 1 mW of transmit power, narrowband. I'll let you know when I have something working up there.
HAZE. Another mm-wave propagation factor to consider is that of haze. There are references describing significant additional losses, when haze is present in the path. Haze is usually non-vaporized water droplets, suspended in the atmosphere. We are working on a graph showing losses attributable to haze. As if we weren't in enough trouble already...hi.
It has really been fun working on the 24 & 47 GHz bands. Here's a list of known activity in my area.
24 GHz: W4RX, W4SW, W3HMS, KB8VAO, K4EFD, K8ISK, K8GP, WA0QII, ND3F, K6LEW, W3IY. The best dx I know of, in this group is 204Km. Signals have generally been very strong on paths out to 174Km. The 204Km QSO was a little weak, but SSB quality. I think haze was adding attenuation that day. We often test over an 82 Km path, and signals seem strong, even in hot-humid weather, so far.
47 GHz: W4RX, W4SW, K4EFD, K8ISK, K8GP, K6LEW, W3IY. Best dx here has been 174Km. We can regularly work a known 82Km path using 100 microWatts on SSB! It probably would be much harder in summertime with high temperature and humidity...haven't tried this yet.
76 GHz: K6LEW, K8GP. best known dx ~ 6ft (hi). We're hoping to try some real dx soon.
Another factor to consider is that of haze. There are references describing significant additional losses, when haze is present in the path. Haze is usually non-vaporized water droplets, suspended in the atmosphere. We are working on a graph showing losses attributable to haze. As if we weren't in enough trouble already...hi.
Many of the operational stations on the mm-wave bands are using DB6NT transverters. These units have been described in Dubus magazine, and much of the information about them is available from http://dpmc.unige.ch/dubus/index.html for free. The hardware can be expensive to buy new, but it is possible to buy a partial kit for 24 GHz directly from DB6NT (Kuhne Electronics), and make something cheaply. All of these transverters are just harmonic mixers on microstrip PC boards, with IF gain, and some T/R switching. Boards are available at a very amateur-friendly price, and the parts are also obtainable. The difficult parts are the beam-lead diodes for the 47 & 76 GHz transverters. These are sold by Agilent for about $20 each, but the catch is you need to buy 50 at-a-time. Someone on the G3PHO website had done this, and was selling single diodes. Other diode suppliers include Metelics, and Microwave Semiconductor Corporation.
One of the biggest challenges to getting on these bands is obtaining a good local oscillator. There are brick oscillators with sufficient RF output power in the 12 GHz range, but most units I have seen are a little low in output. There aren't too many cheap MMICs for use at 12 GHz, without getting out your wire-bonding equipment, and doing some serious assembly work. DB6NT makes a suitable unit, which he sells assembled, or to save dinero, you can build a kit from Eisch Electronics. If you do this, you should probably buy the 12GHz LO kit, less PCB, and buy the PC board direct from Kuhne, since it is a much-improved board with real plated-thru holes. The Eisch kit requires tedious installation of many ground rivets or Z-wires. These LOs have performed well, usually getting us well within 25 KHz of each other on 24 GHz. Even 47 GHz has been pretty predictable, as for frequency, but we usually have to keep one finger on the tuning knob during a QSO. The crystal oscillators are outfitted with thermistor heaters, but they're not as good as a real crystal oven. For serious work, you will want to build an ovenized oscillator, and separately package it, so it can stay out of the wind and weather. It's really nice to have a +17dBm 12GHz LO that runs off +12VDC, as these units do.
Waveguide components have become scarce around my local hamfests, but you can make stuff affordably by purchasing waveguide parts from Eisch Electronics in Ulm, Germany. You can also make rectangular brass tubing work well, but making flanges that mate with other units is a bit challenging. Occasionally useful things show up on Ebay, but there are usually some professional sharks out there scarfing this stuff up for non-amateur prices. Who is Julia Huang, anyways?
If you are good with a milling machine, you can really save some time and expense getting stuff made on the mm-wave bands.
There are also surplus items available from various places from time-to-time. Keep in touch with your mm-wave friends, and see what comes available. Share the wealth...don't squirrel the stuff away in your basement as an investment...we need activity on the bands NOW. If you find a good deal, please either use it, or pass it on to someone who will. The mm-wave hobbyists need all the help they can get!
Soon there will be many more options available, as the commercial world gears up to actually field some equipment on these bands. New Cadillacs, I hear, include some 76 GHz radar transceiver equipment collision avoidance. Maybe you can find one in a junque yard someday. Unfortunately, much of what I have observed is in hybrid-die form, and can't be soldered to PC boards using cheap ham-type construction. If you can find someone with wire-bonding equipment, you will open many new possibilities for building high-quality mm-wave equipment. There are presently some die available for bands just slightly below or above our amateur frequency allocations, and much of these parts appear promising, if stretched into our bands. If you discover something, please share the wealth of knowledge. Who knows...the day may come when you won't have to build both ends of the link to get some QSOs...hi.
73 de W3IY