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Highlights of a weak 2,077 kilometre (1,291 mi) Sporadic E opening featuring 90.7 NBC's Voice of PNG FM and commercial station 93.1 Yumi FM from Port Moresby.

The announcer provides the Yumi ID at approximately the 2:50 and 3:15 positions. Broadcast power is 1 kW whilst the site is 200 metres or 656 feet high (FMscan.org 2012, FMdx.au.com 2010).

What's the point?

This RDX-155 receiver integrating a Silicon Labs' tuner front end was modified to accommodate an external FM antenna. Whilst I should caution that this reception is probably a best case scenario, this video demonstrates this inexpensive device offers some potential for DX reception. The intention of doing such an experiment is to simply learn more about the device.

Suggestions for use

Ideally, connect the existing antenna input to a 1:1 current balun transformer and then to 50 ohm coaxial cable which leads to the rooftop antenna. The balun helps match the unbalanced load to the balanced antenna. That reduces the presence of unmatched currents on the coaxial cable. Matching impedance helps maximize the signal to noise ratio of weak signals.

Potential applications

Travellers who wish to try using this tiny portable tuner for weak signal Es (Sporadic E) reception must be able to plug into a rooftop antenna which is ideally not a UHF-only breed. This antenna used for this particular reception is a modern combination antenna. Specifically this provides full wide band coverage of bands I to V. This weak reception in the video is from the rear of the antenna.

It has been suggested that intense or high powered Es (Sporadic E) reception may permit RDS (Radio Data System) decodes on this device, aiding potential identification.

Soft mute characteristics

Like many modern Digital Signal Processing (DSP) based receivers, the software settings or the AGC (Automatic Gain Control) may be set to mute weak signals. Some speculate this may be one explanation as to why the device seems to exhibit a loud thumping sound occasionally. Or perhaps it is just some residual 'splatter' from the adjacent channel?

The muting of weak signals appears comfortable enough to me. By way of comparison, the soft muting is not as obviously 'aggressive' as the 2007 model Blaupunkt Las Vegas DVD35 (earlier models don't seem to suffer from this ailment) or the 2005 model Sony XDR-F1HD. Those receivers usually require a preamplifier to optimize weak signal reception. Once that's fixed, the receivers provide comfortable weak signal listening for extended periods.

Audio quality

The audio on weak signals with this Silicon Labs' SI4701 chip set seems to invariably be mono. Rather than 'high cut' or 'blend' weak stereo signals, the software has been set so these signals are rendered in mono. Weak FM signals heard on the Tecsun portable radios with the SI4734 chip set to this tuner may be more pleasant to the ear. That weakness is entirely forgivable considering those radios cost at least double this device!

Strong signal immunity

An inline attenuator is screwed into the coaxial cable to minimize spurious images from strong local stations and therefore optimize weak signal sensitivity. Despite the need for attenuation, the strong signal immunity of the Silicon Labs' SI4701 chip set is remarkable. At this difficult location, it totally outperforms a portable radio with the SI4734 chip set (please see the rationale below). In my view, that is the key strength of this device and offsets all areas of potential weakness. That's what makes this device fascinating!

Assumptions & inferences

A casual side-by-side comparison of the data sheets provides some obvious hints. This tuner is quite different to the SI4734 chip set, whilst the selectivity (that is, the ability to readily separate FM stations) remains the same. At least that's one inference based on the 200 kHz specification.

An example of a significant positive difference... The SI4701 chip set used in this RDX-155 has a better IIP3 figure than the SI4734.

Input Third Order Intercept (IIP3) is a determinant of the spurious free dynamic range and a measure of the potential linearity of the amplifier or tuner. Engineers suggest that the better this figure is in tuner or amplifier specifications, the less susceptibility there is to the creation of inter modulation.

A final difference relates to sensitivity. The SI4701 chip set has slightly poorer sensitivity than the SI4734 chip set according to the specs.

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