I've been in Amateur Radio since 1974, and still find new and interesting things to do. I like to build, restore, and operate on the air. This blog has been running for many years, so be sure to check out "Jump to Posts on Specific Topics" in the RH column to drill down and find lots of stuff. Visit www.WB4IUY.net for the lowdown at WB4IUY. Email me at firstname.lastname@example.org if you have any questions.
This is a few more photos of the new repeater antenna build, and some observations.While tearing down a 4-bay antenna on 154mhz for parts & materials, I measured things with my MFJ Antenna Analyzer, and took note of some dimensions...
First, the larger dipoles are from the 154mhz antenna. I made a little pigtail connector for the MFJ, and took some readings. Resonance and impedance of these types of antennas is impacted by proximity to the mounting mast and the width (these are 3") of the loop. They also require at least a 1/4 wl of mast above and below for proper operation. The larger units were resonant at about 154mhz, where I expected. These dipoles are 17" per side, from the mount to the end of the loop, total 35" tip to tip (counting the 1" dia mount in the middle). The loop is 3" wide, edge to edge, and they are 3" from the mounting mast to the edge of the dipole loop. These are the dipoles I will modify for 2 meters.
The smaller dipoles are 15" per side, from the mount to the end of the loop, total 31" tip to tip (counting the 1" dia mount in the middle). They were resonant at about 168mhz. I altered the formula to calculate the proper dipole size, and it clearly doesn't track as I alter the dimensions, so I'll be doing a bit empirical testing (read trial and error, hahaha) to work the bugs out of the 220 and 440 antennas.
Another interesting observation...using DB Products RG-11 cable, the matching sections were pretty much the same length as the tip-to-tip dimension of the folded dipole elements. We'll see how this works out when I get these tuned and installed on a test mast...
I plan to use the single mast section on the right (with the rain bonnet) to install a pair of folded dipoles for 2m, 220, and 440. My plans are to keep this a compact antenna. I realize it'll be a compromise antenna, but I need to keep size down for a DC grounded tri-bander.
Shortly after getting the 442.400 repeater back on the air in test mode, I discovered the receive preamp was not working. This is an ARR P432VDG GaAsFet rx preamp. It has worked well for many years, having originally been installed back around 1994. This repeater took a lighting strike when it was installed at the Zebulon site, and I had never tested the preamp. I had to replace the PA at that time, so I'm guessing that's when the preamp failed.
I put it on the IFR and spectrum scope, and discovered that it was a "deamplifier" :-) About 34db of loss when powered up, the GaAsFet has failed. This unit is about $100, so it's certainly work repairing. A new FET is ordered, so I'll post more about this during / after repairs. In the photo below, I set the sig gen for 0dB direct, and with the preamp inline, you see the 34dB or so of loss.
I posted info on my old Motorola Micor UHF repeater on my blog, and people asked questions about how to do it. This is a fairly easy task, though it's far easier to stack two of these radios (one for receive, and one for transmit), but I enjoyed the task of repeating within a single radio.
There's adequate isolation to do this, even when using a receive preamp, with a good set of cavities. These radios are cheap as dirt, often costing less than the crystal required for conversion to the ham bands. The nice thing, being crystal controlled, is you have no phase noise to deal with as with synthesized rigs. They are TOUGH AS NAILS! I've run them hot, it's been hit by lightning, and has operated an horrible physical conditions at one of the repeater sites. Users have locked it up for hours on end while cross-banding HF operations through it. The repeater in my previous blog has been in operation since 1994, so 26 years is nothing to sneeze at. I'll snap some pics of the internals that I've modified, when I go back into it, to help illustrate the process.
As I said, it's much easier to simply tune up a couple, one on the RX frequency and one on the TX frequency, wire a controller, and let'm fly. But, if you want to modify one for full duplex repeat, here's the info. I wrote the compilation of notes I accumulated and included my experiences in Microsoft Word. You can download the file, and if you don't have MS word, there's a host of online converters you can open it with for free.
Go to my website at http://www.WB4IUY.net , mouse over the toolbar across the top of my webpage to [WORKSHOP], and click on "Schematics Manuals". You'll find Motorola Micor in the listings. Have fun!!
After a long period of inactivity, I'm in the process of getting all of my repeaters cleaned up, checked out, minor repairs made, and readied for going back on the air. Here's a couple of photos of the 442.400 repeater, online in my work shop. I originally built this back in 1993. I've made a few adjustments, and so far, all is running good. The PA is running at low power (15 watts), and it is carrier access. I do have it set up for a sub-audible tone of 88.5, but that is disabled at the moment. It's running on a test antenna out behind the shop, only about 20 feet off the ground.
This will be going back on in the stack of repeaters: 29.620, 53.07, 147.39, 224.80, & 442.400. I'll post more of the permanent installation, once the antenna work is completed.
This repeater uses a 6 cavity Sinclair duplexer on top (light brown enclosure on top of the radio), a homebrew bandpass cavity (gray in color) stacked on top of the duplexer.
The Sinclair duplexer was originally used in the commercial
service a few mhz above the 70 cm amateur band. They were repaired and retuned for the input frequency of
447.400 and the output frequency of 442.400. The bandpass cavity was
built from a scrap 800mhz analog cell site cavity. The repeater was built from a Motorola Micor mobile radio used
by the NC Forestry Service on about 452/457 mhz.
The controller is mostly homebrew, using a Communications Specialists
CW ID'r to handle identification chores. All timing (time-out, COS, tx hang timer, ID front porch timer, etc.) is
analog and built from 555 timers and discrete components. Audio processing is also homebrew using LM324 op-amps.
All audio mixing and controller circuitry is self-contained within the Motorola Case, making for a compact UHF
repeater. A pl of 88.5 is re-encoded on it's output signal during COS activity using Selectone encoders. This
is for linking purposes.
This is probably one of the most popular HF Power & SWR Meters Heathkit ever made. You'll pretty much always find these at every hamfest. Today, they sell for as much as $75, and I've bought some for as little as $10. They're very easy to repair and calibrate, and work nicely on the ham bands from 160 through 10m. I've found them to be fairly accurate (certainly usable) on 6m. Good up to 2000 watts, and can measure SWR with less than 10 watts excitation. I currently have several of these. This is one I bought back around 1991, and use it with my Heathkit HW-101 transceiver.