70cm DATV RX Experiences

[M0ATV]

Just wondering how most people are finding 437Mhz DATV RX, while 23cm datv is very active up here in the IO83 square, 70cm datv is still struggling, myself & Mike G1LWX have been active for well over a year on the 70cm band, but besides ourselves ( who at around 25 miles apart ) who can work each other on very low power ( 1-5 w ) we are stuggling to find regular stations who can RX our signals, this week we have been experimenting with G3UVR ( Heswall, Wirral ) who is a dedicated VHF/UHF worker & we are finding the results on 437 Mhz a bit of a damp squid, some nights he can RX our signals very easily down to a few watts, & others he cannot RX anything up to 100w, this is without any changes being made to the TX or RX setups which are as follows:

My TX> DTX1 19 Element Tonna 5-100w output on 437Mhz
G1LWX > Digilite TX 75w output on 437Mhz
G3UVR RX > SUP2400 with filtered 70cm pre amp into 432Mhz 9 element antenna

The path between myself & G3UVR is very good with 23cm datv stable & workable down to extremely low pwr, but the 70cm path seems to be highly volatile with rapid changes from night to night, I would be interested to hear of other peoples experience of 437Mhz RX good or bad & what changes if any made any difference to your RX on this band, up to now we are finding it a struggle up here & it is potentially putting off other 23cm ATV users around here from trying out 70cm.............Tony M0ATV Stockport

[G8GKQ]

Hi Tony

Have you had a look at the input to G3UVR's Sat RX (after the SUP-2400 - watch the line volts!) with a spectrum analyser or even an rtl-sdr? One of the problems that I used to have was very strong signals just out of (the 70 cm) band that were well within the passband of the satellite receiver. Even with filtering they were blocking the 70 cm DATV signals.

Dave
G8GKQ

[M0ATV]

Thanks Dave, I'll mention it to Dennis tonight during more 70cm tests, he has access to a S/A & we did briefly check it WITHOUT the sup2400 inline to see if there were any nearby blocking sigs, but there didnt look like there was any noticeable big signals present, out of curiousity how did you get around the blocking signals with the sup2400 or did you ditch it for something else.

[M0DTS]

Remember the Sat Rx will 'see' probably a minimum of 10MHz bandwidth, probably more like 20MHz (even when set to 2Ms rates) from the SUP-2400 so need to look on a wide span, lots of strong data stuff on 442 here locally.
Try attenuating the level into the receiver by some 10dB to see if it helps it to 'accept' the strong signals without flattening it.

Locally Fylingdales blocks KM's 437MHz input with it's in-band signals on 70cm at every 1MHz up the band +/- 100KHz..... So 70cm is difficult to say the least here!
The only clear slots are 436.1 to 436.9 and 437.1 to 437.9

Rob
M0DTS

[G8GKQ]

My 70 cm DATV tests used a tuned preamp followed by a Toko helical filter and an SBL1-X DBM if I remember correctly. It was before SUP-2400s were used.

Dave

[g8gtz]

Hi Tony

I know it has been said already but it does sound like interference on 70cms - I've suffered from this both at GB3HV where I ended up fitting a cavity notch filter (see http://blog.gb3hv.com/#home.2 ) and at home where I've got really strong PMR channels from the primary user within the 70cms band. These really hit the 437MHz DATV system even though they may be a couple off MHz of channel and I've put in some notches to keep them out.

70cms goes really well - see Peter G8DKC's comments http://www.batc.org.uk/forum/viewtopic.php?f=24&t=3304 but only with good filtering before the SUP2400...

73s

Noel - G8GTZ

[M0ATV]

Thanks Noel, will check out the above, many thanks.

[G4EWJ]

We finally fixed the problem with the 70cm beam at G3NWR and received G1LWX. The 'No signal' icon popped up every second though.

We put the analyser on the beam and saw some carriers in the 441MHz region going on and off at suspiciously the same rate. The sat receiver will almost certainly have these in its passband and the DG0VE converter may not have rolled off far enough by then.

G3UVR, a couple of miles to the south, but higher, is receiving G1LWX much better with a filter, so that's the next step.

Tony M0ATV - the converter is still up at the club, so give it a try if you're around this week.

Brian

[g8gtz]

Good news Brian,

On the other hand I am now suffering from a trunked radio system recently installed by the Primary User which is actually between 436 - 438 MHz (not going to declare the actual frequency here and when the control channel is on, it wipes out 437 MHz reception at my remote site.

It also hits the 437 MHz input on GB3HV - clearly as it is "on frequency" interference we cannot filter it out, but we have found by moving up to 438 MHz (no need to retune the Comag SL30) it works OK. When it is Txing, you can clearly see it on the GB3HV Tutioune monitor http://blog.gb3hv.com/#post23

I guess these are the joys of being a secondary user but please be aware that during lift condx (remember those!) you may have to shift to 438 MHz to work stations "down south".

73

Noel

[M0ATV]

What is the situation regarding switching to 438Mhz TX, I am having horrendous problems here when beamed to the W ( G3NWR & G3UVR direction ) it completely disables the neighbour across the road in that directions' car entry start up system, also causes problems to his "wireless" central heating control system, all appparently using the 433Mhz band cheapo systems, I'm using 2 inline duplexer systems to filter the 437Mhz UHF sig, & have also put inline a final 437Mhz tuned filter with a 2Mhz bandwidth, but still causes overload to the neighbours system, by switching to 437.500 TX I was then able to open my car of which I was unable to do so before, so by moving up a bit from 437Mhz it definitely makes a difference, will there be any knock on effects to the other 70cm band users while using 438Mhz TX, is this even allowed, was thinking it may infringe on the SAT section of the band, also as Brian G4EWJ has already mentioned, by inserting a 437Mhz tuned filter inline, it made a huge difference to seeing DATV on the band possible to G3UVR, when removed it was impossible even when we ran high power, the filter is not ideal, slightly wide at 8Mhz bandwidth, but at £12 a shot worth a try.