Roundtop Info

Located @ 1100 ft. elevation, Roundtop is our main/hub site which has the most equipment of all the sites and it covers urban Honolulu. We work and live in this repeater's main coverage zone. In terms of connectivity, this site serves as the cross connecting point for the Eastbound link (to Koko Head, Haleakala & Hilo), Westbound link (Kaala & Lihue), Northbound ROIP (Kaneohe) and AllStarLink (run as a private node). It also links to our downtown analog remote receiver.








Here's a list of what's all in the rack from top to bottom (frontside):

- Quantar UHF repeater w/110 watt PA & AC high power supply

- S-COM 7330 Controller

- LDG Electronics RVS-8 repeater voter

- Behringer MX-882 audio mixer/splitter

- Custom E&M logic bridge (contact us if you're interested in building your own).

- PD-526 Six Cavity Duplexer

- GLB Electronics Preselector/Preamp

- Samsung Monitor

- Multi Voip Box(removed since picture); Telex IP-223 for 4 wire audio w/ E&M to Kaneohe site

- Computer (for AllStarLink)

Rack (backside):

- Back of the Quantar

- Dummy load w/ -20 dB sample port

- Quintron QT-7795 UHF Amplifier 250 watts (Find the rare manual for this beauty HERE)

- Custom Panel for 24 volt PA Power Supply (Lambda LZS-750-3) & Fan Temperature Control Thermostat (Ranco ETC-112000-000)


Sinclair SC381-HF2LDF(D00), 6 dBd gain, 18 deg. beamwidth, 230 MPH wind rating. The main feedline up to the antenna is LDF5-50 7/8" heliax. Heliax is used exclusively on all cabling up to the duplexer.


All of the links (except the ROIP) are done over microwave links (carrier grade) using the VF-5C 4 wire E&M cards in an Intraplex server. Audio quality is outstanding due to using 64 kbps u-law encoding and out-of-band E&M signaling.

AllStarLink is a whole other realm which I won't get into here. We only use this for the occasional link to a select few repeaters who are also running as a private node. It also allows us to use the iaxRpt PC client software to access the network from a computer.

Technical Discussion:

In a few ways, the XO system is old school- this was done on purpose. The most notable of these old school ways is that we link most all of our repeater system using 4 wire audio cards on T1 connections over microwave. In this age of IP connectivity, most system operators would opt for a VOIP solution for all their links (like IRLP, Echolink or AllStarLink). While these are great systems (we especially like AllStarLink - we even have our private node using this system) they all require using PC hardware. PC hardware is inherently less reliable than the carrier grade cards we use because of build quality and moving parts. The intraplex server and VF-5 cards are proven reliable for many many years of continuous service without a hitch because they were designed and built with reliability in mind.

In addition to using more reliable hardware, 4 wire linking cuts down on the equipment and power consumption at each site. All that's needed at each site is an audio mixer and an E&M bridge instead of a PC and all its associated hardware. Each site already has an Intraplex server by default.

The Quantar is a staple of the land mobile world. It's a neatly packaged, solid, reliable unit that's highly flexible. In our case, it's configured to route analog traffic to our S-COM controller and route digital P25 traffic internally. P25 does not link externally at the moment, because we are somewhat purists. If you're on P25, we want it to be digital all the way to the end radio. Many hams have linked their Quantar using a DIU-3000 which converts the A/D and then send it out over AllStarLink. While this is a totally workable solution, you do lose audio quality in the multiple A-D/D-A conversions which happen using this method. Ideally, you only want to have one complete cycle of conversion for the best quality audio. Since it's running standalone P25, we get great sound from radio to radio.

The S-COM 7330 controller is a great controller with an outstanding value. It's very well executed and has many useful features. In addition to the typical controller functions, we have it configured to control the link on/off, and disable/enable remote receivers individually. There is an active user support group for this controller in case you have difficulty making it do tricks. One of the more notable touches are the removable jumpers to configure the radio logic interfaces hi/lo and audio path gain stages. This makes setting it up a breeze.

The LDG Electronics RVS-8 repeater voter looks like it has (sadly) been discontinued. It sold for about $300 when it was in production and is a slick piece. It performed the same function as many costly commercial units for a tiny fraction of the price. The voter decides which analog receiver is getting the best signal and routes the audio accordingly. It uses the signal-noise ratio of each receiver to determine best signal (some older voting systems used a system of tones coming from each receiver which indicated relative signal strength). It's connected between all the receivers for this site and the RX input of the controller. It also has a logic interface to the controller which allows us to remotely disable individual receivers.

The Behringer MX-882 mixer serves as the audio cross connect between link audio paths. It allows us to connect an East and West bound audio circuit together while mixing in the local repeater to the link bus and extracting link audio and sending it to the local repeater. It costs just over $100 and is a bargain for what it does. One important modification we made on these mixers is soldering a 620ohm resistor across pins 2 & 3 of the XLR connector (inputs only). Since our VF-5 4 wire audio cards use a 600 ohm impedance, it's important to match the impedance otherwise you'll get very inconsistent audio levels and sub-par low frequency response.

The Custom E&M Logic Bridge is a piece that was designed by Bob NH6XO and built by Mitch KH6MP. This items serves as the logic cross connect between link paths. It also allows us to use legacy VF-5 cards which only supported Type I signalling where -48V was required to activate the M line on the card. The board consists of a bunch of 2N2222 transistors, a bunch of 4N25 opto isolators, a bunch of resistors, a few caps and a voltage regulator. This was initially one of the most troublesome pieces because it was totally custom and I built the first units using "bent lead" style construction. We later got custom PC boards making construction much easier.

The PD-526 Duplexer can be found in many places, including that auction site. It's been through a few changes in brand names and cabling but the design remains the same. It's one of the best UHF duplexers out there because it has fairly low insertion loss (1.0 dB) and T/R isolation of 100 dB. It handles up to 250 watts, which as it turns out, is a perfect match for the high power Quintron PA. Our previous duplexer wasn't quite up to the job when I installed the high power amplifier. I had desense to the tune of about 4dB which is totally unacceptable. There is now zero desense with the 526. I found this unit in our pile of surplus repeater stuff. I ended up recabling it using mil-spec RG-214 because it was such an old version that they had actually used RG-8 for the harness. Once I saw that RG-8, I knew it had to go, especially at 250 watts. It took me a few hours, but I reclaimed all the N connectors from the RG-8 and painstakingly prepared the RG-214 (which wasn't cheap mind you!). I found this handy chart some nice guy prepared of Celwave Duplexer Cable Lengths.

A few notes on tuning duplexers: The best duplexers are Bandpass/Bandreject types. When retuning to different frequencies, always adjust the bandpass (tuning rod length) first and then the bandreject (capacitors). Repeat until best results are achieved. I do all my tuning using a tracking analyzer setup and adjust for lowest VSWR on the bandpass adjustments and deepest notch (highest insertion loss) on the bandreject adjustment. I then verify insertion loss on-frequency to make sure it's within spec. Since test equipment typically has a near perfect 50 ohm impedance (and actual radios do not), I tweak the bandpass tuning ever so slightly after the duplexer is installed (Thanks to Bernie, KH6IAH for pointing this out to me when I first started out). You see, the duplexer also acts as an impedance matching transformer between the equipment and antenna. In the case of this particular repeater, the PA has built-in forward & reflected power metering so I tweaked the bandpass adjustments on the TX side of the duplexer to obtain the lowest reflected power/highest forward power throughput. I verified power through the duplexer by using my Telewave 44-AP wattmeter on the heliax exiting the building. When I first installed the duplexer (after tuning it on the test gear), I was getting about 20 watts reflected to the PA. After tweaking the bandpass adjustments on the TX side of the duplexer, I got the reflected power down to just 1 watt: that's a very low VSWR with 250 watts forward. One interesting bit- since we are running high power, you could actually watch the meter readings slightly change as duplexer components & cabling heat up when you key down for the first 30 seconds or so. I made sure to key down for a good minute before tweaking the bandpass adjustments.

If you don't have built-in metering on your PA, use an external inline wattmeter and measure reflected power. Of course, just inserting the wattmeter between the PA and duplexer changes the effective impedance the PA sees but you'll get close enough. Incidentally, it's usually not worth tweaking the RX side of the duplexer as the slight impedance mismatch does not have a discernable effect on received signal strength. Remember, a repeater is all about squeezing every last fraction of a dB out of the entire system. It takes some time dealing with the nuances but it's worth it. This repeater is a serious performer and has some incredible coverage. Bob has been able to sit in the waiting area of the Lanai City airport and talk in on P25 with no breakup using a handheld on a rubber duckie. I've been able to get in on my mobile from Haleiwa and Laniakea beach.

The GLB Electronics Preselector/Preamp is attached directly to the RX port of the duplexer for minimal loss. This is a bit of a unique receive accessory and is now made by another company . It not only has a preamp but also has a preselector which improves out of band rejection. From what I've read, the GLB unit was one of the best with Angle Linear being the top. It was factory ordered on our frequency over a decade ago. When I put it on a tracking generator in 2012, I found that it needed a bit of touch up tuning which was easily done with two adjustments. Overall, it nets about 6dB gain on frequency. For our system, it definitely did boost our coverage a bit more. You always have to use caution with preamps- sometimes they do more harm than good. Remember, you can't hear below the noise.

The Telex IP-223 unit provides the ROIP (Radio Over IP) link to Kaneohe. It is commonly used in commercial and public safety installations and has many built-in provisions for integrating with radio systems like support for COR and PTT. Other nice features include a built in test tone to set your transmit levels, built-in audio level metering for receive, physical potentiometer audio level adjustment plus electronic pot settings on web interface,

The Quintron QT-7795 UHF Amplifier is a favorite of mine. I had been wanting to get a high power PA for quite some time and one finally came up on auction that was within reach. I bought it right away (using buy it now) because it was pretty darn cheap (about $350 I recall). I really couldn't find much info about it online and just went with my gut and the info the seller posted- It turned out to be a real gem. All I really knew was that it was working, output 250 watts, input 0.25 watts, on MHz and took 24 volts DC input. I had an appropriate 24 volt power supply lying around, so at least I had that part covered.

Quintron is of course long since out of business so finding any documentation was not working out on the internets. I managed to figure out what current company to call (low and behold they are in QUINCY, IL) and they were very nice to pass along a PDF of the manual which details how to align and tune the amplifier as well as specs. Being ever so thankful, I sent them boxes of chocolate covered mac nuts. The Quintron PA is beautifully thought out and very modular. In addition to built-in VSWR protection, it has forward/reverse RF power metering and individual DC current metering of each amplifier stage. Each stage is also tunable with an air variable capacitor.

The PA was missing its original fan & shroud so I ended up making a new one from aluminum sheet and mounted a 120mm ball bearing fan in it. To make it slick and take care of the power supply mounting, I fabricated a custom panel that holds the Lambda switching PSU and the Ranco temperature controller (The original PSU that came with the paging terminal is a real boat anchor). I mounted the temp. sensor to the hottest part of the PA heatsink and set the control to turn on at 90F and off at 85F. Having the fans on temp control keeps the dust collection and noise to a minimum. I also mounted two 120mm fans to the top vents of our cabinet to get that hot air out of the box. Even at full power key down for multiple hours, heatsink temp never rises about 110F. Ultimately, I set the DC PSU for 26.5 volts per the manual (PA uses ~28 amps at full power).

Since the PA only needs 0.25 watts of drive, I dealt with that issue by setting the Quantar to output 25 watts and connected a dummy load with a -20dB sampling port. I then used the sample port to drive the PA.

As for the Antenna & Feedline, we always strive to use the best. I've found monoband antennas work the best on repeaters and there is no substitute for Heliax. We always use a minimum of 1/2" Heliax or 7/8" if the run is long (75' or more). If you want a top performing repeater, don't even bother with flexible antenna feedlines because the loss is much higher and longevity not even close. In addition, some foil/braid shield configurations can also contribute to intermodulation. Go and scrounge some Heliax from a surplus source (just go buy the connectors).

Last Updated 6/16/14