RadCom May 2020, Vol. 96, No. 5

Regulars What next for your IC-9700, part 2 Last month I discussed some of the issues you need to understand when switching to that ‘third band’, 1.3GHz, on your shiny new IC-9700 – or when you connect up your new 23cm transverter. The issues mainly concerned loss and noise and I worked through some examples of the effect of masthead preamps on system performance. This month I’ll look at how propagation works on 23cm, what you might expect and how it compares with the lower bands on your new radio. Next month I’ll conclude with a look at options for practical implementation of a masthead preamp at 1.3GHz. What can I expect to work on 1.3GHz? Let’s start with the similarities between 1.3GHz, 432 and 144MHz. They are all bands that rely mainly on tropospheric propagation enhancements for terrestrial working. There will be no Sporadic-E or meteor scatter opportunities on 1296MHz, at least with regular-sized stations, but aircraft UHÁHFWLRQ LV WKH PDLQ SURSDJDWLRQ PRGH RQ 0+] IRU GD\ WR GD\ '; The overall path loss via the troposphere is made up of free space loss, GLIIUDFWLRQ DQG WURSRVSKHULF VFDWWHULQJ 'HSHQGLQJ XSRQ WKH SDWK OHQJWK and obstructions, one of these three will dominate. There is a really good propagation tutorial on all this [1] written by Mike, G0MJW, which should be required reading for any amateur interested on how his/her signal gets out. I’m a big fan of making rough but sensible calculations to get a feel for things, so let’s work through a few examples to get a feel for path losses. “Calculator on”, as James, G3RUH would say! Using G0MJW’s path loss calculator [2] and checking the (roughly) 24km line of sight path from my QTH to the local beacon site, GB3CAM, diffraction is shown to be the dominant propagation mode. Total path losses for 144, 432 and 1296MHz are calculated at 121, 128 and 134dB respectively. This shows the 6-7dB change for each tripling in frequency, so this indicates free space loss with a little diffraction. If we are using a typical 8-ele Yagi on 144MHz, an 18-ele on 432MHz and a 44-ele on 1296MHz, the antenna gain increases some 4dBi for each band. This means that the 1.3GHz signal will be only 3dB or so lower than the 144MHz signal, not 6 or 7dB, for the same transmitted power. Now, if we look at a much longer, over the horizon path, such as the 403km from my QTH on the Fen Edge to beacon GB3MCB in Cornwall, troposcatter [3] becomes the dominant propagation mode as there is multiple diffraction over hills and, due to earth curvature, obviously no possibility of line of sight. Adding to this, atmospheric losses begin to affect the higher bands. The total path losses for 144, 432 and 1296MHz are now calculated as 201, 216 and 230dB respectively. That’s about 14-15dB increase for each tripling in frequency, not unexpected because troposcatter path loss follows a frequency cubed law. The loss increase going from 144 to 432 is 10log(432/144) 3 = 14dB. See Table 1 for a summary. Link budget Looking back to last month’s column we used the VK3UM calculator [4] to work out the sensitivity of a typical receiver with and without a masthead preamp as -147 and -137dBm respectively. Looking at the 403km path from here to Cornwall, if we take a 1296MHz station with 10W (+40dBm) and a 44-ele Yagi with a gain of 20dBi we have an EIRP of +60dBm (~1kW). 7R ÀQG WKH H[SHFWHG UHFHLYH VLJQDO VWUHQJWK VXEWUDFW WKH WURSRVSKHULF VFDWWHU path loss of 230dB then add the receive antenna gain (assuming the same type, so 20dBi). We end up with received power of -150dBm. If we run that calculation again but at 144MHz, with the calculated path loss (Table 1) and 100W into a typical 8-ele Yagi, we have +50dBm Tx power plus 12dBi antenna gain giving an EIRP of +62dBm (~1.5kW). Subtract 201dB path loss and add 12dBi for a receive antenna and we end up with a received power of -127dBm. What we haven’t considered is that the horizon noise level at 144MHz might typically be 15dB higher than at 0+] VR HIIHFWLYHO\ RXU G%P ÀJXUH LV HTXLYDOHQW WR G%P giving a similar signal to noise ratio as at 1296MHz. Remembering also that we’re comparing 10W on 1296MHz to 100W on 144MHz, we can expect similar distances to be worked on these bands with similar powers. Tropo openings Once the bands start to ‘open’ and we get tropospheric ducting, all bets are off and the normal troposcatter calculations go out of the window. Because of the geometry of ducts, 1296MHz signals tend to be much stronger than they would be on 144MHz. Back in the March GHz Bands I reported that Keith, GU6EFB was 59+ here on 1.3GHz at 369km off the back of my beam. He in turn reported that the signals on 1.3GHz were “unbelievably strong”. This is not unusual in big tropo openings, where you can effectively be presented with a ‘1296MHz waveguide’ to take your signal well over 1000km with close to just free space loss. Waveguides act as a high pass ÀOWHU GXH WR ZKDW·V NQRZQ DV FXW RII IUHTXHQF\ [4] and, with the right duct size, 144MHz can be below that cut off and just not propagate. Summary Path losses are subject basic losses, statistical variation (QSB) due to PXOWLSDWK DQG WR DLUFUDIW UHÁHFWLRQ HQKDQFHPHQW $OO WKLV VKRZV WKDW WZR well-sited stations with just 10W should be able to make troposcatter QSOs on 1296MHz at close to 400km at any time. Use a digital mode and/or add D : DPSOLÀHU DQG WKLQJV EHFRPH HYHQ EHWWHU If you have anything to add to the GHz discussion, email me or tweet @g4bao and @ukghz using the hashtag #GHz_bands. Websearch [1] www.mike-willis.com/Tutorial/propagation.html [2] www.mike-willis.com/software.html [3] www.vk5dj.com/doug.html > @ KWWSV ELW O\ +]3;% [5] https://bit.ly/2TGp4iJ GHz Bands John C Worsnop PhD CEng MIET, G4BAO john@g4bao.com 64 May 2020 TABLE 1: Comparison of paths and losses on different bands. Band Path length Diffraction loss Troposcatter loss Dominant mode 0+] NP G% G% 'LIIUDFWLRQ 0+] NP G% G% 'LIIUDFWLRQ 0+] NP G% G% 'LIIUDFWLRQ 144MHz 403km 229dB 201dB Troposcatter 432MHz 403km 254dB 216dB Troposcatter 1296MHz 403km 280dB 230dB Troposcatter

RkJQdWJsaXNoZXIy ODQxOTY=