Saturday, 15 June 2013

S-Meter

Some meter settings from service manuals.
Yaesu FT8900:
1 dot = -5dBuv
Full = 23dBuv

Yaesu FT817:
S1 = -3dBuv
Full = 22dBuv
On FM only

Alinco DJG7
S1 = 0dBuv
Full = 20dBuv

Icom IC2800
2 dots = 0dBuv
Full probably around 15dBuv

Saturday, 1 June 2013

More antenna results

Full results so far




Mobile / Handheld Antenna Test

Tested some more mobile and handheld antennas using my RTL TV tuner dongle in the car.
There was also a Moonraker 2m/70cm mobile antenna which I'd bought recently and which is now getting used as my main mobile antenna (see earlier blog post).

1. Straight whip with PL259 connector, a 1/4 wave on 145MHz
2. Nagoya NL77B 2m/70cm PL259 mobile
3. Nagoya NA702 2m/70cm SMA flexi whip. This isn't resonant on the amateur bands at all, it seems to really be tuned to around 165MHz, making it more useful for scanners.
4. Watson Regular Gainer BNC flexi whip. Sold as a scanner antenna, it is tuned to 150/450MHz.
5. Watson WSMA-7000 2m/70cm flexi whip.
6. The stock antenna from a Uniden Bearcat UBC3500 scanner. 

The USB device shown is a cheap TV/FM/DAB tuner based on a RLT2832U and R820T chip design. It cost about £7 and can tune from 24 - 1800 MHz in all modes and bandwidths using RTLSDR software. The SDR# software has a spectrum display with a dB scale which I used for this test.
I set the receiver gain so that the strongest signal in the SDR bandwidth (it converts down to a 2 MHz bandwidth for the software to process) was around 10dB below full scale to make sure nothing was being overloaded.
All antennas were tested on a magmount on the car roof. The car roof makes it slightly off vertical polarisation but at VHF it probably won't make much difference with these short antennas.
The coax shown in the photo is the original (75 ohm?) stuff which came with a short TV antenna whip on. I just soldered a PL259 plug onto it.
 Here are the results, I took readings on 2 different bands, around 140 and 164 MHz. These give a good signal source as there are some nearby transmitters operating 24/7. On 140MHz I had the RF gain turned up full without overload. I turned it down a bit on 164MHz. The USB tuner is off frequency, usually by about 45ppm but I didn't bother correcting it for this test.
All readings are in dB relative to whatever I got with the 145MHz 1/4 wave.

Photo Antenna 140 MHz 164 MHz
0 Moonraker Mobile -1 -1
1 Straight Whip (50cm) 0 0
2 Nagoya Mobile -2.5 -1.5
3 Nagoya 702 SMA -6 0
4 Watson Reg Gainer BNC -5 -10
5 Watson 7000 SMA -10 -23
6 Uniden UBC3500 SMA -15 -9

The straight whip is the best performer, but isn't suitable for UHF. The Moonraker and Nagoya mobile antennas are similar on UHF but the shorter length of the Nagoya is probably letting it down at 140MHz.
Antenna 3, the Nagoya 702 flexi whip, is shorter than any of the mobile antennas but is equal or better than the larger antennas at 164MHz. This is in line with what we see from the SWR readings of this NA702, it also shows that the mobile antennas are not really that good up at 164MHz if they can be beat by a 29cm long whip.
At 140MHz the short length and 165MHz tuning of the NA702 is showing, with -6dB gain. The Watson Regular Gainer is even shorter at 21cm but has 1dB more gain on the lower frequency. The extra loading coil needed to get it working on 150MHz is narrowing the bandwidth a bit though, with -10dB gain on 164MHz. It's marketed as a 25-1900 MHz wideband antenna but the NA702 (which doesn't look like it could have any loading coil inside it) is actually a better choice if the 8cm extra length isn't a problem (try the Watson Regular Gainer around 200MHz - it's hard to tell if it's even plugged in!).
The Watson 7000 SMA antenna is very poor at 164MHz, probably because it has been carefully tuned to the amateur bands despite its small size, and this makes it useless for anything else. Even the Nagoya NA702 and Watson Regular Gainer (neither of which are resonant in the amateur bands) will be better than this on 145MHz. But it is one of the shortest and this is going to limit the bandwidth for anything you could transmit into.
Lastly, the Uniden scanner's stock rubber duck antenna isn't very good at anything (it's just as bad on UHF), but it's the smallest. I'd guess it was designed for about 160MHz.

For the car, I'd choose the Moonraker if I was looking for something compact.
For short under 30cm handheld antennas I'd choose the NA702 even though the tuning is a bit off.
Another time I'll test some larger mobile and handheld antennas.