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The Heil Parametric Receive Audio System (PRAS), and ideas about conducting antenna feeders through windows.


Tim Kirby casts his reviewer’s eye over the Heil Parametric Receive Audio System (PRAS), shares ideas about conducting antenna feeders through windows, and responds to exciting news from readers.

Reviewed by: Tim Kirby 


It all started, as usual, with an email from the editor! Would I be interested in taking a look at the Heil Parametric Receive Audio System (PRAS, Figs. 1 to 4)? I confess that Google was pressed into service immediately, to find out exactly what the PRAS was and what it did!


Fig.1: Front panel and controls on the Heil PRAS.

Fig. 2: Back panel and connections on the Heil PRAS.


Fig. 3: The Heil PRAS HPS-5 powered speaker.                                                                                                                                                                  Fig. 4: The back panel of the Heil PRAS HPS-5 powered speaker.

It all sounded interesting, and I noted that the intention was to improve audio, particularly speech, to make it more intelligible. I was intrigued, particularly when I saw the price tag of around £475. How much difference could the unit make, and are we all going to want one of these at our receiving station?


The first thing I always do, when I look at a piece of equipment like this is to take a look at what the manufacturers say and what they perceive the benefits to be. This is what the Heil website has to say about this unit: “The Heil Parametric Receive Audio System (PRAS) is a state-of-the-art audio system that enhances the internal audio systems of amateur radio, shortwave, commercial, and CB receivers. The PRAS allows radio operators to modify, shape and improve the audio output of their receivers using three separate equalization controls. Mid-range frequencies are the most critical for achieving clear voice articulation in receive audio.


“The PRAS allows operators to have unique control over these important frequencies. First, operators can adjust the parametric midrange filter, from 400Hz through 4kHz, with the recommended ‘sweet spot’ at 2.5kHz. In addition, operators can control the presence of these midrange frequencies, plus or minus 15dB. Combined with a low-frequency filter set at 160Hz and a high-frequency filter set at 6kHz, the PRAS provides operators unparalleled control and quality of their receive audio.


The webpage goes onto to describe the connection possibilities in and out of the unit.


You can read more on this website;


The unit arrived in a most satisfying large package. It was well packed and immediately oozed the quality feel that Heil are renowned for, albeit at a price to match. Unpacking it, there were two units, the first one was a powered speaker, and the second one was the control and interface unit.


A brief but entirely adequate set of instructions described the setup of the PRAS and the various interfacing possibilities. A power supply is also provided. It runs both the speaker and the control and interface unit, by means of a splitter-lead.


So, what does it sound like? The first thing that I connected up was probably one of the cheaper receivers in my shack, the very useful Uniden UBC72XLT handheld scanner, which normally sits on the windowsill and is monitoring local (and not-so-local) airband traffic. On its own, the audio from the scanner can sometimes be a bit indistinct and muffled, depending on how strong the received signals are, of course.


I plugged this into the PRAS by means of a 3.5mm jack plug and lead, which is supplied with the unit. After this, I had some simple setup to do. I tweaked the input level, to match the output of the receiver and I played around with the audio frequency controls to emphasize speech frequencies. This was very simple and quick.


Although I was a little bit cynical about the price of the unit, I have to say that the results, even with the cheap scanner, were superb! The combination of the powered speaker and the filtered speech provided very clear audio indeed, making for a very pleasant listening experience overall.


Transmissions that would have seemed indistinct on the scanner’s speaker offered a very clear copy, and the squelch tail from the scanner was softened to a very pleasing and gentle level.


Although the filtering of the PRAS is intended to emphasize the frequencies of human speech, I wondered what musical programs would sound like through the system. I suspected that the majority of listeners would, at least occasionally, listen to programs with at least some musical content. I hooked up a cheap DAB receiver, which had an audio out socket. The audio through the PRAS was a distinct improvement on the internal speaker on the DAB radio.


I also tried a couple of short wave receivers, including my venerable Sony ICF-2001 from 1981, and without exception, the received audio was much improved.

You will most probably feed audio into the PRAS by means of either a 3.5mm jack or a one-quarter-inch jack plug. The PRAS comes with a 3.5mm-to-one-quarter-inch jack adapter.


I was a little surprised that there wasn’t more than one audio input on the PRAS, to enable operators to switch, for example between a VHF scanning receiver and a short wave receiver on the unit itself. If you wanted to do that, you could buy (or build) a switch box, or even use some sort of mixer. The ‘low-tech solution’, of course, is to move the input lead to the PRAS from one receiver output to another!


The powered speaker that comes with the PRAS is the Heil HPS-5. It’s a 24W, two-way, low distortion amplifier, using a titanium midrange tweeter and a low-frequency, speaker.


The connection between the PRAS and HPS-5 is made by means of an XLR-to-XLR cable. The HPS-5 comes with a mounting bracket, should you wish to install it on the wall or similar. It sounded and looked good on the operating bench too.


There’s a receive output on the front of the PRAS allowing you to connect to a recording device. In addition to this, there are two independent headphone outputs (3.5mm) – each with its own volume control.


Something that was important to me, which may or may not be important to you, is how the PRAS reacted to RF levels in the shack. I had no breakthrough or hum from the PRAS from my 50, 144 or 432MHz transmissions. All my HF aerials are down at the moment, so I didn’t get the chance to try that.


Overall, I was impressed with the PRAS. The audio quality is excellent, and it makes a big improvement on most receivers. It comes at a substantial price, and I think it will probably appeal mostly to enthusiasts who like the best of everything in their listening post or shack.


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My thanks to Peter Waters of Waters and Stanton Electronics for the kind loan of the Heil PRAS. The PRAS costs £469.95 and is available through Waters and Stanton.


Waters & Stanton Ltd, Unit 1, Fitzherbert Spur, Farlington, Portsmouth PO6 1TT


Tel: 01702 204 965


E-mail: [email protected]


Flat Coaxial Cable

How do you get a coaxial cable through windows? Readers may remember a question from Martin R a few months back, wondering about techniques for passing antenna feeders through windows and window frames when it is not possible to drill holes. Well, Martin has answered his own question. He writes:


“I have an antenna on a balcony. Getting the antenna lead indoors was not a problem in summer, when the lead simply came through an open window. It’s a different matter when the weather turns colder; sitting in a cold room is unpleasant at the best of times. 


“I wondered if there was anything as exotic as flat coax available. To my surprise there is! A search on the Internet, Amazon revealed that you can get an F socket with flat ribbon coax cable:


“I ordered one, and it works. Because the flat coax is designed for satellite installations, it has F sockets on either end. I got around this by buying a couple of coax satellite leads with F plugs from Screwfix. I cut them to the length I wanted and fitted PL259 plugs on the other end. I could then use an in-line connector to connect them to various adapter leads I have made.

  “I couldn’t find any technical spec for the flat ribbon. However – using an SDR and comparing the ribbon against a coaxial cable lead through the open window – there appears no degradation of signals across the VHF and UHF part of the spectrum.

This sounds like a great solution! It might work very well on receive, and I might be tempted to use this for low power transmissions too!


Listening Post

It was great to hear for the first time from Dipole Dennis! DD wrote, “Sitting in the warm, and using an SDR, I managed to discover another channel connected with the town DMR Shop Watch system. I have a Whistler TRX-1, which can be set up to record audio. I initially set it to record for one day, and the results were interesting. It seems that as well as Shop Watch, it is also used at times for covert surveillance by other professional-sounding types; information passed is kept to a minimum.


“At first, I was unable to find the base channel. However, I have a small program (‘Duplex’), which calculates the channels splits. By using this, I was able to find out that the base channel, which the mobiles are in contact with, is the main town Shop Watch channel.


“One thing I have noticed is that the Shop Watch mobile signals don’t appear as robust as FM signals on the antenna I use. The DMR mobile signals drop out more. Oh, for a better antenna position!


“I decided to set up a very simple traffic analysis. Out of interest, I set up a spreadsheet at the end of each day, recording how many times the channel was used. At the moment, there is no set pattern.


“I’ve not yet got to grips fully with the TRX-1, I’m not sure how to set the Trunking facility or the Scan function. I’m sure that, if I can set up the scan with the main Shop Watch channel and the mobile, I could then record both the mobile and main channel.


“I have also found what appears to be the traffic Wardens’ channel, and I am not sure whether this is tied-in with the main Shop Watch channel.

I have several retired laptops I can use with SDR Sharp software. Other than the Windows operating system, there’s only SDR Sharp running (this reduces the load on the CPU). This would allow a lot of recording space. Is there any way of looking at sections of a recording other than having to sit through and listen from the beginning of the recording to the end?  


Very interesting, Dennis! Although there are programs you can use to play the files recorded by SDR#, I am not aware of any way of speeding up playback, in order to analyse what you have in the file.


Can any readers help, please?


Here in Oxfordshire, my ADS-B monitoring has continued, and the improved antenna system and low noise amplifier have really improved performance. Despite being shielded by trees in some directions, the results are very good, and I frequently see aircraft out to a distance of 250 miles or so.


I upload my data into the FlightAware system. As readers may know, there is a kind of ‘league table’ of contributors, based on the number of aircraft seen, positions reported and so on. I was excited to see that, following my upgrade of the system here, I was listed in the Top-500 of the league table; not bad going!


Further improvements can be made by mounting the antenna even higher, of course, but I am not certain whether I shall have the chance to do that here. It’s certainly fascinating to see the difference to the previous indoor antenna I used for ADS-B. It would, perhaps, see a maximum of 30 or 40 aircraft at peak times. With the external antenna and low noise amplifier, I might see up to 185 aircraft at busy periods.


With my radio amateur’s hat on, it’s also interesting to see when I suddenly see more aircraft than usual in particular directions, corresponding usually with a tropospheric enhancement.


This article was featured in the July 2019 issue of Radio User

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