Radio

Well, I had a full wave loop for 20 Metres, which I tried to convert into a folded dipole (now this is proof that one must think before one acts) which was a failure as there wasn’t enough wire strangely enough, and to top it off after the strong winds in Melbourne the other night caused some damage to that failed antenna.

Now generally for a dipole we work on the theory that the full length of the dipole is ½λ so for example for the 20 meter band we could go for the following formula:

(300 ÷ 14.175) ÷ 2

where 14.175 is the centre frequency I’m trying to work with.

So from this we get the value of:

10.582 metres

As I’m working with dipoles we can simplify the formula above to just 150 ÷ λ where lamda is the frequency we’re working with. I’m wanting to get good coverage over the 20 meter band so I’m using the frequency of 14.175.

So for a normal run of the mill dipole our two “legs” of the dipole would be:

5.291 metres

Now a folded dipole has the characteristic of offering a better match over a wider band. Meaning if all goes well the folded dipole will work over the entire 20 metre band without having to use a tuner.

Though using a tuner and balanced feed line it is said that one can use the folded dipole on other bands where the half wavelength is at the lowest frequency you’re operating on.

Now for my folded dipole, the full length of it for ease of construction was 10.6 Metres, and the distance between the top of the folded dipole and the bottom (I’m sure there is a word for this but for the life of me I can’t think of it.) was .6 Metres.

Now the feed impedance of the folded dipole is somewhat higher than that of your standard dipole (which is around 72Ω) , and is close to 300Ω so instead of the 1:1 Balun that a standard dipole would use a 4:1 Balun to try and bring the impedance as close to 72Ω as we can.

Anyway, after building this, and putting it up in the air, I’ve checked the swr of the antenna, and it appears to be 1.5:1 across most of the 20 metre band. So I need to tune the antenna up a little, as it is just a touch too long. But it works very well compared to the full wave loop I had used previously, and much better than my first attempt of a folded dipole.

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Well I wasn’t happy with the full wave loop for 20 metres.  While it was quiet, I couldn’t hear much either, and also I was proving difficult to match.  I may not of  thought things through properly when measuring it and may of left it a bit too short.

Anyway I’ve had some luck with folded dipoles previously (on 40 Metres and 80 Metres.) So I decided to build one for 20 Metres.

So far so good, not a perfect match it’s about 1.5:1 across the band.  But that’s not too bad.  I might see what I can do to improve the match.

So far so good. Just listening to someone up on the Birdsville Track.

~Ivy

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Well set up an inverted V dipole for 80 Meters.  And so far so good. It has a 1:1 swr at 3.5 MHz, and about 1.5:1 at 3.6 MHz.

So as I don’t do much in the way of CW, I could shorten the antenna a little, so to get it closer to 1:1 around the area in which I tend to talk on.  Which is around 3.59 MHz for a little group that I like to touch base with.

Also with the help of VK3GNU, who is much better at getting ropes up into trees than I.  We set up a full wave loop for 20 Metres.

Still thinking of what else to build.  One of the things we’re considering is a beam for 20 Meters.  How many elements, and do we buld traps to make it shorter. Who knows but it will be a lot of fun.

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Finally got the 40m dipole up at the new QTH. Today I’m going to call cq and see what I get.

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Well today is the 2010 VK trans tasman contest. Going to be fun for the club at the Sherbrooke community school. Starts at 8:00 UTC today =)

Lots of fun.

~Ivy

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Been playing round with dash code, and have made a little mobile version of this blog. I will set up something to auto redirect mobile browsers to this later.

It’s now available here.

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Well today ACMA released their white paper on the “400MHz band” Which covers 403MHz to 520MHz. As this covers the 70cm band we use (420MHz to 450MHz as a secondary service) this paper effects us as amateurs.

As the ACMA puts it “The The way ahead – Decisions and implementation options for the 400 MHz band consultation paper outlines a number of final decisions for the future use of the band and proposed implementation plans.”

In the original ACMA proposal was that for 403MHz to 430MHz was for the exclusive use of federal, state, and territory governments. This would mean for us amateurs that we would loose the bottom 10MHz of the 70cm band. Though this section of the band (420 to 430MHz) is restricted in some states. For example in the “Melbourne” area the excluded sections of the 70cm band are as follows:

• 420.00–420.75 MHz
• 421.25–424.75 MHz
• 425.25–430.00 MHz

While in the “Perth” area all of 420 to 430MHz is already disallowed.

These exclusions can be found on page 28 of  the Radiocommunications Licence Conditions (Amateur Licence) Determination No. 1 of 1997.

So under this new ACMA paper, all of the 420 to 430MHz section of the 70cm band is lost to the amateurs who could use it.

From 430 to 440MHz this paper states that “The 430-440 MHz rage is used for defense purposes and by the amateur service, including amateur satellite, in line with internationally harmonised allocations.” And ” …hence there is currently limited scope for general use of these services in this band.”

So from 430 to 440MHz There will be no changes.

But for 440MHz to 450MHz there is no mention of amateur services, only the following:

“Spectrum in 440-450MHz will be made available in consultation with the Department of Defense on a temporary basis. Users in this part of the band in high and medium density areas will be required to relocate by 31 December 2012. Users outside of high and medium density areas will be required to relocate by 31 December 2013.”

Here there is no mention of amateurs at all. Which is worrying. But it may be that we are still able to use this section of the band as a secondary service as we do now.

Oddly unlike the minor mentions of the amateur service. There is an entire chapter of this paper dedicated to UHF CB radio.

Unfortunately this document doesn’t really tell us as amateurs what will be happening to the 440-450MHz part of the 70cm band at all.

Edit, just looked at one other part of this document, and it says the following, “Arrangements for the radiolocation and amateur services outside the scope of this plan.” So it looks like we may not lose any more than the bottom 10MHz of the 70cm band.

DE

VK3IVY

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Well Hannah and Myself have moved to a new house in Pakenham. RF wise it’s reasonably quiet which is good.

All we need to do is get some proper antennas up. Right now there’s a 40m dipole (that really needs tuning correctly.) It works okay but really could do something better with it.

~de VK3IVY

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The wia has a pdf of all of the repeaters in Australia. Unfortunately they don’t give this information in formats that might be useful when using a computer to program your radio.

Anyway I’ve now managed to get the data for the 6 metre, 2 metre and 70 centimetre repeaters into a csv file, and an excel spread sheet.

Anyway the excel file is here and the csv file is here. If there’s any errors pleas mention it.

de Vk3IVY

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Well went to the Kynton Hamfest last weekend. Lots of fun lots of junk, and lots of shiny things.

Ended up getting a new HT from the Strictly Ham stall. A Yaesu VX-7R as our old VX-5R had seen better days.

Anyway it’s an interesting little radios, it does 6 metres, 2 metres, and 70 centimetres FM at 5 watts. It’s also got general receive from the AM broadcast band to 999MHz. Also it can out 300 miliwatts on 1.25 metres (not a band we have allocated.) And the fun thing is that it can also do 6 metre AM with 1 watt.

So far a nice little handheld and I have no gripes with it (though the rubber duck antenna is well a rubber duck antenna.)

Also TTS Systems had a nice stall with the beautiful TENTEC radios and the BudiPole range of antennas.

Then there was the Junk and bits and other bits and pieces. Some bits and pieces, and lots of “shiny things”

Nearing the end of the day I went saw Jim VK3AMN, and Ed VK3FEET talk about what we’re doing with the school and I was able to answer some questions regarding the ARISS contact we did last year.

All in all a fun day.

de VK3IVY

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