Tag Archives: Radio

More about the Alligator Hat

The June 2009 edition of AR carries a small article I wrote about lowering the resonant frequency of a HF helical antenna to allow me to use it on a lower frequency than its design centre frequency. My 80m helical, for example, is resonant at 3585 and provides the lowest SWR at that frequency.  At the CW end of the band, say 3520, the same antenna has a SWR of over 3:1, sufficient for the IC706 to cut back its output power to less than 10w.  To operate on the CW end of the band with this antenna I therefore need to deal with the mismatch using an ATU, or change the resonant frequency of the antenna.

The method I used was to load the helical with a capacity hat formed by a short wire, actually an alligator lead I had in my field day accessories box.  I was operating from the car and was nowhere near home at the time.

Here is the SWR curve from the front panel of the IC706, with the radio tuned to 3585 kHz.  The SWR bar-graph is small but the general shape of the SWR curve can be appreciated.  The bars represent the SWR at frequencies below and above the centre frequency indicated by the main frequency dial.  The small dot below the bar in the centre of the graph reminds you that is the measurement corresponding to the dial frequency.  The increment per measurement is 10 KHz, as set using key M2.  The white rectangle drawn around the SWR bar graph was added to the photo by me.

SWR centred on 3585
SWR centred on 3585

After adding the loading and retuning the radio to 3521, the SWR curve moved down the band and here it is centred on 3521:

SWR of loaded helical, centred on 3521
SWR of loaded helical, centred on 3521

A nice application for the SWR indicator and sweep function in the IC706.

John Moyle Memorial Field day March 2009

For this year’s JMFD contest I thought weather and propagation conditions were fairly bleak.
The weather was wet and windy to say the least.  Many field stations reported having their tents and masts blown down.

The high point for me was working 3UHF on 1296 with only a single 18 el yagi, and barefoot (10w nominal).   The distance was 501 km according to the VK1OD distance calculator, using the VK1DA/p and VK3UHF locations from the VHF Logger.

I didn’t like my chances of having hf antennas stay up and didn’t want to extend the tear down process, so I limited myself to the vhf/uhf bands.  I had a car full of antennas and several extra masts but in those conditions, there is no point in trying to do too much.

The temp in the tent at 5AM Sunday morning was 3.5 C though the official overnight minimum according to BOM was 2C.   No wind gust peak data was available.

Operating techniques and problems observed.

There is a continuing tendency for operators to call and make contacts on only one frequency, 144.150.  Can everyone please tell their club operators that there is no repeater there, they are allowed to move the big knob in the middle of the radio panel.  It is ok, nothing will break, the rest of the band also works for making contacts.  It would be better to train vhf ssb operators on HF so they get to know how to operate on ssb, how to work the tuning knob and how to  tune around the band to find stations to work.  FM channels and repeaters are quite the wrong training ground for SSB but I’m afraid that the FM repeater operation mode (staying on one frequency, as if it is the only conduit to any other station) is the method many operators learn and continue to use.

It is up to the experienced operators to teach new operators better techniques.  I appreciated those experienced operators who I heard requesting a QSY as soon as initial contact had been made.

During the contest I tried many times to make contact with some stations in the greater Sydney and Melbourne areas, whose signals were perfectly readable, but whose operators seemed to want to chat to locals interminably, on 144.150.  There are bonus points for working longer distances and these operators were ignoring those chances.  eg. a contact with another local station is worth 2 points, but a contact with a station 300+ km away would be worth 50 points.  This surely would make it worth listening to a weaker signal.

We should encourage people to operate in vhf events in a manner similar to the HF bands.  Find a clear frequency (within the band plan) and call CQ.  If looking for a contact, tune the band.   If activity is low, don’t move too far from other activity (but be mindful of local interference problems – this is why I qsy 30 kHz up from 150, not just 5 kHz as I might on HF).  If activity is high, move further out.  Give the dx something to tune for.  Don’t clump up and make it impossible!

My QSO tally

All contacts ssb.  These scores are about half the corresponding number from the summer VHF/UHF field day in January.

6m – 9
2m – 54
70cm – 31
23cm – 7
13cm – nil.

New antennas for 1296 MHz

I received the two 18 element yagis for 1296 Mhz and they appear to be strongly made. We will see whether they are any better than the previous antenna.  I have borrowed a splitter for them and will use that for the Summer field day in mid January, unless I can make my own before then.

I found a website offering data on stacking distances for yagis.  For a boom length of 1.5m on this frequency the stacking distance recommended is 2.4 wavelengths, which works out at 55cm (23cm wavelength).  The feedlines on the yagis are about 25 cm long but that’s not long enough to reach the power divider, so additional connectors and cable will be needed, eating into the stacking gain.

Next step for 23cm is to configure the power amplifier so it can be operated remotely from the tent.  I read some comments on the UK microwave reflector about power levels from IC910H not being up to the 10w level expected.  Time to get out the power meter and check mine out.

Preparing for spring VHF/UHF field day, November 15/16

This week’s project is to prepare for next weekend’s VHF/UHF field day.

I plan to operate from a mountain southwest of Canberra, with equipment for 50, 144, 432, 1296, 2403 and 10368 MHz.  The first four bands are bands I have used before but the last two are new for my station.  The 2403 equipment will be a simple transverter driven by a FT290R radio on 144.  The antenna for that band will be a grid type dish.

The 10368 MHz equipment has been borrowed from another local amateur.  It is a transverter driven by an Icom IC202 on 144, the antenna is a dish.  It all mounts on a tripod which readily allows azimuth and elevation adjustements.

On all bands the main mode used will be voice, using upper sideband.  For some contacts with more distant stations, morse (CW) will be used as it is much easier to hear weak morse signals than weak voices.

2008 winter VHF FD at Red Hill, Canberra

The equipment was set up and taken down each day as I was not able to stay overnight.  The morning temperature on Sunday was about 5 which was much warmer than it could have been.  Some contacts into Sydney on 2m and 70cm, with an attempted contact on 23cm with Adrian VK2FZ.  Contacts made around town on all bands and a visit from Ian VK1FOTO (later VK1IS), John VK1JST and Charles VK1CM.  Power was from the GMC 950va alternator.