Roland Turner

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9V1RS yagi experiment

As planned, our new duplexer has made it possible for us to resume work on antenna configuration. We're particularly concerned about improving gain and therefore coverage to the east and realised that a short yagi might improve coverage for a large part of Singapore.


We demonstrated in November that a collinear antenna at the upper balcony provides excellent coverage, however the length of the collinear means that it is exposed to lightning risks so installation is likely to require the services of a lightning protection engineer and additional equipment. We worked around this problem by deploying the same antenna on the lower balcony on the basis that if it is inadequate then we'd undertake the lightning protection work required for deployment on the upper balcony. This is starting to look as though it will be necessary. As this process will be time-consuming, we're also exploring other options for improving gain in the interim. Yesterday we tested and switched to a yagi. The results appear to be generally good.

Complicating all of this is that half-duplex tests don't necessarily predict results when operating in full-duplex, as the repeater does. A little over a month ago we finally determined that it was appropriate to replace our old duplexer. We installed the new one about a week ago. This did not solve all outstanding problems of course, but it did set the stage for meaningful testing in full-duplex mode. Naturally we have yet another problem (there's always one more...): the new duplexer appears to have an unreasonably high VSWR. We've approached the reseller to start the process to resolve this and in the meantime have worked around it by inserting a 3dB attenuator in the line between the radio's TX port and the duplexer's high pass port. Quantitatively: Icom specifies that the transmitter not be subjcted to a VSWR exceeding 1.5 while Sinclair also happens to specify that their duplexer will present a VSWR not exceeding 1.5. Inconveniently the latter is currently presenting a 2.3 VSWR, which corresponds to a return loss of ~8dB. Inserting the 3dB attenuator increases the return loss by 6dB to ~14dB (3dB reduction in forward power to the duplexer and antenna plus 3dB reduction in power reflected by the duplexer and antenna), for a VSWR very close to 1.5. This is more marginal than I'd like, but is at least within rounding error of Icom's specification.


In the meantime, we'd like to increase gain on the upper balcony without incurring unacceptable lightning risks. James 9V1YC hit on the idea of using small yagi and I happened to have one in my storeroom, so yesterday Harish 9V1HP and I installed it. As with the various tests performed during the installation of the duplexer, we also conducted a series of blind tests with the following results.

N.B. In contrast to the tests performed on the weekend, most of the configurations tested yesterday are not feasible for immediate unattended deployment, they were included only as data points to help us understand the situation. They are shown with a red background.

Configuration 9V1ZV (pre-amp) 9V1ZV Buona Vista 9V1KT Kallang 9V1KG Bedok 9V1PL East Coast 9M2LXM Johor 9V1CD Bedok
1 S9+1seg S6 (about 5seg/S-unit up to S9) S9+20dB Hiss on background audio. Nothing 0, S0 S9 + 2db
2 S9+2seg S6.5 S9+40dB Hiss on background audio but still better than in (1). Nothing 5, S6 S9 + 3db
3 S9+20dB S9+4seg (10dB/5seg above S9) S9+40dB Full quieting. Excellent copy. S2 (not readable) 5, S9 Full quieting S9 + 4db
4 S9+16dB S9+3seg S9+40dB Full quieting. Excellent copy. No apparent difference from (3). S2.5 Just can hear signal & modulation, but barely readable 5, S9+20 Full quieting ++ S9 + 2db
5 S9+4seg S7.5 S9+40 R5 Some hiss on the background. S3 (between 4 & 5 I adj my antenna pos) R1, S0 S9 + 4db
6 S9+2seg S6.5 S9+40 R5 Some hiss on the background. Audio appears to be louder than in (5) but hiss remains as like in (5). S2 to 2.5 R1, S-1 S9 + 2db
7 S9+20dB S9+4seg S9+40 R5 Very minimal hiss as compared to (5) and (6). Overall better than (5) and (6). S3 5, S9 +20 Full quieting S9 +6db
8 S9+21dB S9+10dB+ (5seg) S9+40 R5 Same as results as in (7). Unable to tell the difference between (7) and (8) S3.5 to 4 5, S8 Full quieting S9+10db
9 S9+40dB R5 Minimal hissing S3 5, S7 Full quieting S9 +1db S3
10 S9+40dB R5 Minimal hissing just like (9). However, overall audio slightly louder S2 to 2.5 5, S6 hissing S9 S3
11 S9+40dB R5 Minimal hissing. No apparent difference from (9) and (10). S2 Hard to read 5, S9 Full quieting S9 +3 cutting out

It would appear that for most reporting stations, the results were excellent. The marginal stations were 9V1KG, 9V1PL, and 9V1CD. For these stations:

The tested configurations were as follows:

graph TB Antenna --- Duplexer Attenuator --> Duplexer Repeater -- TX --> Attenuator Duplexer -- RX --> Repeater
  1. 25W half-yagi 90deg
  2. 50W half-yagi 90deg
  3. 50W half-yagi 45deg
  4. 25W half-yagi 45deg
  5. 50W full-yagi 90deg
  6. 25W full-yagi 90deg
  7. 25W full-yagi 45deg
  8. 50W full-yagi 45deg
  9. 50W unity
  10. 25W unity
  11. 25W half-yagi 45deg (same as 4)



We left the antenna in configuration 4/11 for the time being.


As is hopefully obvious, there is a dilemma here:

10 is a bit of a mystery. It's essentially the transmit configuration that we've been running since the move, but with reception through the duplexer from the same antenna. It looks as though this might benefit from further exploration.

Adding directionality has solved a large problem (coverage for a swathe of Singapore to the east), but at the cost of lost service for at least two previously working stations to the north and south of the repeater. I suspect that the tradeoff is worthwhile, but we need to keep in mind that it is a tradeoff.

Next steps