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Amateur Radio Repeater Repair & Site Visit

Sep 032019

 

Today I traveled to the site of an amateur radio repeater on UHF (70cm) that I maintain after a fault on the transmitter appeared and gradually got worse over the past few months. Lesson learnt - always bring EVERYTHING you may ever need!

VK7RHF low power mod

Oct 232016

Having removed VK7RHF's transmitter to test at a solar powered site, it was necessary to test standby current consumption. Basic current measurements were made. 750mA was measured as standby. 13A while transmitting (including three fans that run while transmitting and for 5 minutes after PTT finishes).

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All repeaters are running 24/7, therefore standby current can be a problem if excessive, more so then if compared with current consumption while transmitting. For example, one hour of transmit time a day at 14 amps means we need to put those 14 amps back into the battery. In addition we have a standby current of 0.75A x 23 hours = 17.25 amps. So regardless of transmit activity, every single day we are going to take at least 17 amps out of our battery. Reducing transmitter power usually does not save a lot of current as the PA will be more inefficient. Therefore we want to reduce standby current.

A good way to do this is to switch off the power to all unnecessary bits of the repeater until needed. We need the receiver and repeater controller to be active all the time for a start, but we can turn off the exciter, PA and microcontroller from the converted MLS low band radio. I used an idea that Dion VK7DB had come up with. He used a SUP75P05-08 P channel FET as a high side switch on the power line to the PA board. These particular FETs allow 20A of continuous current without a heatsink, but were discontinued some time ago. More readily available were SUP53P06-20 from Jaycar. We want the resistance between the drain and source to be as low as possible. RDS according to the datasheet was 0.0195 ohms at VGS = - 10 V. Maximum Power Dissipation at an ambient temperature of 25 degrees is 3.1W. We know I = √P/R. 3.1/0.0195 = 158.974. The square root of that is 12.6. Therefore at 25 degrees and not on a heatsink, this device can handle 12.6 amps as an absolute maximum. With a TC=25 degrees (case temp - heatsink) it can dissipate 104.2W, or handle 73 amps. Plenty.

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Here is the circuit. It's fairly self explanatory. We use the fan control in the NHRC controller as this pulls low to GND when the transmitter keys up and for 5 minutes afterward. We put a pull up resistor from +12V to the gate to make sure the FET is in a known state until triggered from our fan control. When the fan control goes low, our FET turns on and switches power on to our PA/exciter stage of the repeater. 

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Orange wire is our logic from the fan controller. Resistor is 10K I think. It works. 

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Current consumption when running the receiver only is now 140mA. A saving of 610mA or 14.64 amps over 24 hours! 

More simulcasting with the RTCM/VOTER

Sep 152016

Further to previous posts about the KL Reference 12MHz to 10MHz and Simulcast Experiments, I had finally got around to purchasing a BG7TBL GPS receiver module due to some motivation in a video by Joe KC2IRV who is doing the same thing.

First order was to modify the KL.... I had a spare 6M conversion laying around which I decided to modify further for this purpose. 

The original 12MHz TCXO was removed. 

7DB low power mods were then performed. I had decided to do this as potentially the repeater may be located at a solar site. It also removed all the congestion of the main board. I removed the receiver and decided to make this a TX only unit. 

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There is a bit going on here. The voter board is on the left. Next on the right, is the 9.6MHz OCXO needed to keep the clock into the RTCM/Voter stable. The other OCXO is the 10MHz which I relocated into the box.  

Wired in the Allstar RTCM/Voter board. Fairly straightforward as all that was required was power, discriminator audio, transmit audio and PTT. Squelch action is done in software. I may need to run CTCSS decoding in the future, so there is an option for that. The RTCM/Voter board can generate CTCSS encode in software too. I had issues with getting enough TX audio level out of the board on my first trial. The voter board can generate enough TX audio level to drive the radio to 3KHz deviation, but is maxed out at around 4KHz using the 5-Tone modulation input.

On advice from another who uses the RTCM I ran the TX audio into the Encode input of the TX module. I also shorted R340B (27K) in series with this line to remove attenuation and probably some high frequency roll off and so that the audio is direct to the modulator. The Allstar software takes care of pre-emphasis and deviation limiting. 

I then programmed the EPROMS for 10MHz reference use as documented earlier. This proved to be straightforward after writing down what had been done previously. 

I placed this transmitter at one of the sites and ran my test unit on my bench at home on low power to create an overlap area close to my house. 6 metres propagates well, so I had to turn the power down on the exciter to around 40mW. I zero beat the transmitter on the bench to match the one on the mountain. When generating a CTCSS tone, I could hear what seemed to be the two tones beating out of phase with one another. Audio was distorted and or chopped up in some parts of the overlap, especially when stationary. I then had a look at the time delay between the transmitters and my receiver. Radio travels at the speed of light, or 5.4 microseconds per statute mile (1.6km). It worked out that I was really close to my TX at home, but far from the TX on the mountain (20km). This is a delay of 67.5uS. Above a delay of 83uS with a 1KHz tone, things start to degrade. At 2KHz the max is 42uS.

You can read more about the theory here - http://www.simulcastsolutions.com/userfiles/file/simulcastforums/technology_Simulcast_Theory_F20.pdf    

The VOTER boards have a Simulcast Launch delay function which enables a delay of one or more of the transmit audio times to suit users in an overlap area to make sure the audio arrives in phase and in time. I adjusted the delay of the TX unit at the mountain site to around 330 (67.5uS). This made some improvement, but the audio was still tearing and poor in some areas.

Next I decided to have a look at the audio response of the KL unit on my bench from 300Hz to 3000Hz. All audio needs to be in time, but also in phase, deviating up and down the same on each transmitter. I setup a reference tone of 1KHz to equal 3KHz of deviation and plotted the response. These are the results.

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I made a mistake in my initial measurements. The low end of the audio frequency range deviates higher. This is because my test sets HPF was set to 50Hz, so an accurate measurement was not made. But it is corrected later - further on that soon. The CTCSS Input was my audio input, used on both transmitters at the time. As you can see it rolls off rather sharply after about 1800Hz. There is a LPF after the clipper where the audio was being injected, so I decided to adjust this to see if I could get a better response. I could. You can still see some rolloff after 2300Hz, due to the LPF, so it wasn't perfectly flat. Any variations here between the transmitters would cause additional distortion. 

I then modulated directly on top of the varactor diode on the VCO board. You can see pretty much a flat curve. I then redid these measurements with the proper HPF selected in the test set. In addition, I also lifted R403 on the VCO board and injected audio there. I couldn't modulate from the VOTER board directly onto the diode as it caused the VCO to fall out of lock. The level was also way too high anyway, therefore injecting at R403 gives me a bit more range to set levels. Schematic below of this.

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As you can see, low frequencies are more flat now - although there is some variance due to the HPF being directly at 300Hz in the HP8924c. The level should stay constant (within 10%) of 3KHz deviation across the whole audio frequency range. As you can see it rolls off a little bit more that 2700Hz at a 3K tone. For now, I'm modulating at R403 as it's much flatter, and will be easier to get the other transmitter matched to this. That is the next step and is where I am up to on this project. 

 

 

 

 

VK7RVP gets 2m

Aug 072016

On 6/8/16 VK7's DB, ML and HH traveled to St.Valentines Peak (VK7RVP).

The purpose of the visit was to check battery voltage levels, take measurements, install a 2m repeater and check a potentially faulty antenna system that the previous digipeater was running off.

Several different routes to the summit were explored. The usual routes bridge was washed away due to recent river flooding. Another further down the road had a locked (normally open) gate. Another potential entry point was checked but this also had a locked gate. These three tracks would of allowed driving to the bottom of the main track, and saving a further 30 mins of walking time each way.

It was decided to have a go via the public track. We left the carpark at 12.30pm, much later than anticipated. A river crossing was required, although the usual log that is used had washed down the river. Luckily an alternative crossing had been made. The summit was reached at 3.30pm by 7HH, shortly followed by 7DB and 7ML. The start of the track indicated a 4 hour return trip.

During the walk, we crossed paths with a pair of walkers who passed us on the way up, and near the summit. It was asked if they had a flashlight we could borrow in case we lost light and return to them, to which they replied they did. As they were locals it was advised by 7HH that they discuss with 7DB and 7ML further down the track about the location to return the torch. 

Conveniently the walkers forgot this, and failed to mention it to Dion or Mat. In any case, 5 mins was taken for a breather and photos at the summit. There was no wind and the weather was clear at this time.

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Dion and Mat started working in the hut installing the 2m repeater whilst I undid the coax seal tape on the potential faulty folded dipole. Testing in the hut with the MFJ analyzer showed an erratic SWR curve. Testing at the antenna however showed that the antenna was fine, with an R of around 43 and an X of 0 at 146.725MHz, an SWR of 1.16. The next suspected issue was a polyphaser in the hut which was replaced. The anaylzer still was not reporting what was read direct at the antenna, so on the next trip the feedline will be checked. In the meantime it was reasonable enough to run the repeater on.

Halfway through work we were circled by a light aircraft with one of a local amateurs mates piloting. Later we received aerial photos of our expedition, albeit blurry. 

Dion and Mat found bad cells in the battery box, so these will need to be fixed on another trip. The 2m repeater was switched on and test reports came in. We left the site at 5pm, with the weather changing....

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 Light was fading and the remaining hour of the walk back to the car, including the river crossing was made with the assistance of our VX-8 handheld radios LED light feature and smartphone flashlights. We were able to depart again at 7.15pm.

 

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