Normally I review radios separately, but this time I will make an exception. Both the Puxing PX-UV973 and the Wouxun KG-UV8D are a new breed of Chinese radios. Both have two independent VFOs, both are cross-band capable. In part one I will discuss some of the gory details, but focus a bit more on theory.
In order to be cross-band capable, you need two independent VFOs. The idea itself is far from new: the Standard C520/C528, manufactured from 1989 until 1993, could do it too. The concept of independent VFOs and cross-band repeat capability was abandoned in favor of models with only one oscillator, which were much cheaper to produce. Modern radios are still dual-band, but can’t listen to both bands simultaneously, nor do cross-band repeat. Instead we got dual watch radios, which switch between both bands on regular intervals to check for activity.
Not only are dual band / dual watch radios much cheaper to produce, you will also avoid a number of technical pitfalls. Cramming two transceivers into one package is the easy part, making sure these two transceivers won’t influence each other in some negative way is another thing. That’s hard, very hard, especially when you combine two amateur bands like 2 meters and 70 centimeters. Ever calculated the third harmonics of 145 MHz?
Cross-band capability makes sense in a number of situations, but in a hand held? A hand held is supposed to be just that – a portable transceiver. In the best of circumstances your little (negative gain) antenna will be about 2 meters above ground, in the worst case scenario (clipped to your belt) around 1 meter above ground. Cross-banding from such a low altitude is rather pointless.
In order to put the system to good use, you will need to connect the radio to an outdoor antenna, preferably as high above ground as possible. In that respect a cross-band system is no different from a normal repeater system: antenna height is everything. While this will ensure that range problems will be solved, it might introduce another problem: receiver overload. Whether this is a problem or not depends on a lot of factors (design, RF pollution in the area), but so far all hand helds I’ve seen can’t match the filtering of most mobile radios.
Problem 1: harmonic suppression
If there’s any situation in which you don’t want to see harmonics at all, it is in these cross-band capable radios. To my surprise both Puxing as well as Wouxun nailed this.
– Puxing PX-UV973: -76dBc on VHF, -65dBc on UHF
– Wouxun KG-UV8D: -65dBc on VHF, -64dBc on UHF
Third harmonics and above undetectable.
Problem 2: sensitivity
A cross-band capable radio needs much better RX filtering. Better RX filters however will also introduce a certain amount of insertion loss. Because of this – and assuming both manufacturers did it right – I expected these radios to be less sensitive than we got used to.
– Puxing PX-UV973: -120 dBm on VHF, -120 dBm on UHF
– Wouxun KG-UV8D: -122 dBm on VHF, -121 dBm on UHF
To put this in perspective: there have been a number of radios reviewed here what could pull of an astounding -129dBm. This means that both radios discussed here have sacrificed about 6dBm in order to get the filtering right. Before you look at these numbers in disgust: less sensitivity is far better than crappy filtering.
Problem 3: preventing both oscillators of interfering with each other
Here both radios fail miserably. For some reason both manufacturers weren’t able to to get this right, resulting in an awful lot of spurious emissions up and down the carrier.
The problem is most evident when you’re receiving around the third harmonics of your transmit frequency. Because my test frequencies are always 145 MHz and 435 MHz (mid-band for European hams), it was impossible to miss.
This is how 145 MHz signals looks like on the spectrum analyzer:
This image is valid for both radios. When you change the RX frequency, the distance between the peaks will change. When you lower the RX frequency enough the peaks will eventually disappear from the screen. Before you think that the problem is gone — nope. As soon as you adjust the frequency span of the analyzer, they’re back in town. The peaks got smaller and smaller eventually, but never disappeared completely.
This behavior is a problem, a really big problem. If you prefer clean signals under all conditions, these radios are not for you.
(to be continued)