DTT/analogue tests with a real aerial

So how does all this pan out in practice? What can you expect of a straight aerial/DTT or TV combination and the same with the addition of a 22dB booster amplifier with a 5dB noise figure? Does the booster do what most people want the to do - allow you to still get reliable reception using a much poorer aerial?

I used a Maxview 14 element (similar to the current A1002/WQ) aerial mounted outside. Given that my location is over 15 miles from the transmitter this is not really suitable for DTT, and it is not a CAI benchmarked aerial. However, it gave satisfactory DTT reception at this site with a 9dB noise margin in this test. The aerial's main advantage was it was to hand. My main aerial system is for analogue and uses a channelised distribution amp mounted in the loft it would not be a fair or useful test to use that to test a typical DIY DTT rig. The aerial has radio line-of-sight to the transmitter.

I simulated the effect of poorer antenna installs by adding attenuation after the aerial. Under normal circumstances this would be an insane thing to do in a near-fringe area but allowed me to explore the performance limits. The aerial was fed, via the attenuator followed by Labgear MSA262 amplifier (5dB noise figure, 22dB gain using full output) , to a Sony VTX-D800U receiver. The loop-thru output was then fed to a Sony SLV-E7 VCR to test analogue reception. Attenuation was added to the straight feed to the receiver first. Measurements were taken of BBC2 on channel 44.

I did not have the gear to do digital channel power to hand, so this has to be seen as a test of a typical almost-fringe DTT reception area. It confirms the basic design of the transmitter power levels, in that the aerial is satisfactory to deliver minimum signal level on analogue with a little bit in reserve, and the same (wideband) aerial at the same location provides satisfactory DTT reception with a little bit in reserve.

Attenuation (db)	Rx signal ch44 dBuV	notes	Amped RX signal	Amped notes
0	63	digital and analogue OK	NA
6	57	digital and analogue OK	79	digital and analogue OK
9	54	digital and analogue OK	77	digital and analogue OK
12	51	dig lost vision, sound burbles analogue OK	75	digital and analogue OK
15	-	-	72	digital lost both S & V
18	45	analogue noisy but still usable	68	analogue less bad
24		limit of what is acceptable		limit of what is acceptable

Here the Sony VTX passthrough did nothing for the analogue signal, which had a fixed pattern noise at very low signal levels. Which is fair enough - it is not designed to work at such levels.

Even so, we can draw two conclusions.

The 22dB booster bought us just 3dB of breathing space as the aerial feed worsened
Analogue is still usable some 10dB after the DTT signal is lost

with a set-top aerial 3dB is less than the level variation you get as people move around in the room, so the 3dB gained in the test is not really all that much practically. And it is nothing like the 22dB gain of the amplifier!

Picture quality with decreasing signal levels

experiment demonstrating boosters don't compensate for aerial deficiencies

Improving noise figure with boosters

Selecting and siting your TV aerial

Why DTT is different from an analogue install

electronics home

Source: CAI website, July 2005, typical range £80-£190
Set-top aerials are not usually balanced, so you will pick up more interference on the cable than you would with one designed for the job. It'll still be a lot better that on top of your TV!
Run, do not walk, from anybody trying to sell you a UHF TV aerial with a parabolic dish on it. To be effective the diameter of the dish needs to be at least one wavelength, which is 0.5m at 600Mhz for the middle of the UHF TV band. That is one BIG set-top aerial. Any dish at UHF TV frequencies is marketing frippery, not engineering.
60dBuV minimum, 80dBuV maximum into 75 ohms if you want to be pedantic.