Amateur Radio and Digital TV RFI 2

couch-potato-1224537-mI was recently talking to a neighbour and he asked casually if we were experiencing any loss of signal with our digital Freeview TV reception. I said we had (the XYL had mentioned it) and he went on to say that he had received a leaflet through the post saying TV reception may suffer due to the roll out of 4G mobile internet in the UK.

I didn’t really think too much of it until something else he said resonated (excuse the pun!). He explained it seemed worse at weekends and during the evening. This unfortunately fitted in quite well with my operation, so i thought i’d conduct some tests to see if the problem was me or indeed the 4G.

First of all the TV aerial we use  is shared across the whole apartment block. So if it was affecting me and my neighbour then it was also likely to be affecting the rest of my neighbours not good at all!

The TV aerial is quite closely located to my multi-band dipole, my dipole being inside the loft and the TV Yagi outside about 5 meters away.

Is it me?

My plan of action was to test transmission on all the bands i frequently use (40m/30m/20m/17m/15m) at the maximum power allowed by my 2E license (50 watts).

What was i aiming to establish? Firstly, was i causing breakthrough on all bands or was it one in particular? Secondly could i reduce the power to a level where the problem didn’t occur?

I turned on my TV and my ICOM-7100, both of which are located in different areas of the house which presented a logistical issue of being in two places at once. The XYL was useful here as she could watch the TV, leaving me to experiment in the shack.

Digital TV such as Freeview behaves quite differently to analogue TV when it encounters interference. It’s very much an all or nothing thing, you either get a perfect picture or you get pixalation, freezing or indeed nothing at all.

I decided to put out some extended CQ calls using PSK31 which would also allow me to witness first hand what was happening at the TV end. The CQ calls lasted about 20 seconds giving me enough time to run from the shack to the living room while the transmission was in progress. The other reason for using PSK31 is that it’s a full duty cycle mode, so unlike SSB, as soon as you start transmitting it uses full RF power and doesn’t vary like SSB does on speech peaks. Something else to note is that breakthrough comes and goes with modulation which is another reason i chose PSK31 as an extreme candidate for testing, as it produces constant modulation.

I first of all tested on 40m, 30m, 20m and 17m. These presented no problems to the TV reception at 50 watts, so i seemed to be in the clear on these bands. I only had 15m left to test so i was starting to think that maybe it was the mobile 4G interference after all. It was then that i started a test transmission on 15m at 50 watts.. bingo! This caused the TV reception to be completely lost during the transmission. Now i had something to work with.

With transmissions on 15m at 50 watts being established as the problem, it was now time to test if reducing the power would prevent the breakthrough from occuring. I first tried 10 watts, started the CQ calls and then running to the living room to observe the TV. 10 watts didn’t seem to present a problem so it was time to step it up to 20 watts, this was also fine. Cranking it up to 30 watts saw the problem came back and the TV reception was lost. Getting closer! Dropping the RF power to 25 watts also caused the issue. It was time to drop it down to 20 watts and test for an extended time, at this power the problem did not reoccur.

During all of this testing i actually had five QSOs on various bands while testing! This was useful as it helped me triple check it really wasn’t a problem on the given band.

Next Steps

With these casual experiments i established that the RFI was only a problem on 15m at power above 25 watts. This is especially useful to know, as i now know how to stop this from happening and will not exceed 20 watts on 15m , leaving the XYL and neighbours to continue enjoying uninterrupted digital TV.

But why was this happening in the first place?

The Cause

The cause of the breakthrough was very unlikely to be harmonics, 15m is on 21MHz and the TV broadcast band starts at 450MHz. This would mean that it would need to be the 21st harmonic of 21MHZ – incredibly unlikely!

The TV antenna itself is unlikely to be picking up the signal on 21MHz – if it was 70cm that was causing the problem then it would be possible but not at 15m. Two possibilities remained here, it was either being picked up on the TV antenna feeder (on the outside of the coax) or it was the TV mast-head pre-amplifier.

TV mast-head pre-amplifiers are particularly prone to overload from nearby transmitters. They have a very large bandwidth (below 86MHz to above 850MHz), with little or no filtering at all.  So it could be fourth/fifth harmonic of 21MHz or intermodulation products.

I can’t get to the amplifier to fit a notch filter and i also can’t replace the amplifier with a filtered/screened version. Additionally i can’t move the dipole or TV antenna so choices are limited. My temporary solution is to lower power to a maximum of 20w on 15m. I’m going to use this new enforced power limit as motivation to explore some digital modes which are excellent at low power such as JT65, Olivia and WSPR.

I’ve recently built another multi-band dipole from speaker wire for bands above 15m including 12m, 10m, 6m and 4m. So i’m now keen to find out if the RFI is limited to just 15m or whether it covers other bands above 15m. I’ll be sure to repeat the same tests when i’ve put the dipole up. I’m hoping that it is limited to 15m but currently have no way of knowing as i haven’t been active on anything higher.

Further Information

The causes of RFI and their resolution is something that is covered in some depth during the foundation, intermediate and advanced stages of the UK licensing scheme. Funnily enough i had just begun studying EMC for my advanced license the same week!

The RSGB produce leaflets that are a good source of EMC information (http://www.rsgb.org/emc/) and can really help with these cases. Fortunately it looks like i don’t need to ask for their further assistance as long as i keep to my newly imposed 20w limit.

2 thoughts on “Amateur Radio and Digital TV RFI

  1. Reply Paul Dec 19,2013 10:57

    Hi James. Can you get access to the mast head amplifier? Sometimes they are installed in the building rather than right at the antenna. If you can I would suggest trying fitting a braid choke in the TV aerial downlead at the point it enters the masthead amp. It’s easy to do, requires no modification to the masthead amp and will not affect the TV reception at all (other than possibly fixing the problem and so improving it).

    Make a choke by winding a length of TV coax round a ferrite rod or through a ferrite core. Fit a coax plug and a coax socket at each end (F type or Belling Lee, whichever the masthead amp uses). The length isn’t especially critical but you need enough turns (inductance) to look like a choke at 21MHz. A ferrite rod wound single layer should do it.

    Something like this: http://upload.wikimedia.org/wikipedia/commons/0/0c/Homemade_balun.jpg

    or this:

    http://www.mds975.co.uk/Images/amateur_radio/Line_Isolator_internal_m0ukd.jpg

    But wind as many turns as you can fit in the rod or in the core. Clip on ferrites can look attractive but won’t have enough inductance at 21MHz unless you put lots of them on the cable.

    • Reply James Stevens Dec 19,2013 13:12

      Thanks Paul, this is very handy. I may have to do some detective work, as i think each flat may have their own amplifier unit which obviously makes things a bit more tricky!

      73s James M0JCQ

Leave a Reply