Sirio GPA 60-108mhz

[piratefm]

Has anyone successfully tuned on of these 1/4 antennas?

https://www.sirio.store/index.php?main_ ... cts_id=212

The included cutting chart seems to indicate that all of the adjustable elements need to be the exact same length. However, I’m struggling to get a low SWR because soon as you adjust one element, mainly the vertical whip it immediately gives a high SWR to the point my transmitter VSWR alarm came on.

So, I left the main centre whip (vertical element) at the length the chart said, and I then found it easier just to extend the 3 elements around the base. But now them 3 lower elements are much longer than the centre whip but the SWR is perfect.

But I don’t think this is how it should be?

I don’t want to keep turning the transmitter off/on making adjustments each time as I’ll be here all day

[radionortheast]

I don’t think it really matters if the radials are shorter or longer than the main element, i’ve seen ones where you only ajust the main vertical element, suppose it would be easier than ajusting 3 or 4 radials. I think what matters is the total lenght say of one radial and the main element, so 108Mhz would be 132cms total, radials would be 66cms and a main element 66cms, you could make one shorter or longer than the other by about 5-10cms without it doing much. So you could make the main element shorter 63cms, the radials 69cms for 108Mhz, the thickness of the elements may make it even shorter. Anyway the fact the bottom radials are longer than the top vertical element shouldn’t matter, I used use an aerial that had 69cms radials and the main element was 61cms, it worked perfectly on that frequency.

[piratefm]

I don’t think it really matters if the radials are shorter or longer than the main element, i’ve seen ones where you only ajust the main vertical element, suppose it would be easier than ajusting 3 or 4 radials. I think what matters is the total lenght say of one radial and the main element, so 108Mhz would be 132cms total, radials would be 66cms and a main element 66cms, you could make one shorter or longer than the other by about 5-10cms without it doing much. So you could make the main element shorter 63cms, the radials 69cms for 108Mhz, the thickness of the elements may make it even shorter. Anyway the fact the bottom radials are longer than the top vertical element shouldn’t matter, I used use an aerial that had 69cms radials and the main element was 61cms, it worked perfectly on that frequency.

Thanks for explaining, most helpful! I shall have another look at it when it’s not so bloody hot!

[Albert H]

There is no point in just tweaking and hoping! You MUST use a proper SWR bridge (at the very least). Just relying on an "SWR alarm" on your transmitter runs the risk of severe damage to the gear!

The proper instrument for aligning aerials is a proper "Antenna Analyser". The MFJ259 used to be a standard tool for most VHF (and HF) "ham" operators, allowing construction and calibration of antennas before they're ever connected to a transmitter. Take a quick look at https://mfjenterprises.com/collections/mfj-analyzers and you'll see the kind of thing you need.

(Incidentally, we always used to joke that MFJ stood for "Made From Junk"!).

An "Antenna Noise Bridge" is also a good tool for quick antenna alignment. You can build one yourself for £15 - 20. You need a good quality receiver for your transmission frequency to use with the Noise Bridge. Ideally, the receiver should have a signal strength meter (the bargraph ones aren't accurate enough).

If you're going to be serious about getting your aerials right, you can't just "hack-and-hope"!

[piratefm]

There is no point in just tweaking and hoping! You MUST use a proper SWR bridge (at the very least). Just relying on an "SWR alarm" on your transmitter runs the risk of severe damage to the gear!

The proper instrument for aligning aerials is a proper "Antenna Analyser". The MFJ259 used to be a standard tool for most VHF (and HF) "ham" operators, allowing construction and calibration of antennas before they're ever connected to a transmitter. Take a quick look at https://mfjenterprises.com/collections/mfj-analyzers and you'll see the kind of thing you need.

(Incidentally, we always used to joke that MFJ stood for "Made From Junk"!).

An "Antenna Noise Bridge" is also a good tool for quick antenna alignment. You can build one yourself for £15 - 20. You need a good quality receiver for your transmission frequency to use with the Noise Bridge. Ideally, the receiver should have a signal strength meter (the bargraph ones aren't accurate enough).

If you're going to be serious about getting your aerials right, you can't just "hack-and-hope"!

Sorry I should have pointed out I am using an SWR Meter. The Sharman AV-200. The problem was that I needed to keep turning the transmitter off, adjusting the elements and then turn the transmitter back on, recalibrate the SWR Meter and take the reading. My concern was turning the transmitter on/off would eventually cause some sort of damage to it? It takes around 10secs for it to lock to the frequency too. Just was taking a fair amount of time.

The VSWR alarm only comes on when you touch the elements to adjust when transmitting lol.

[Albert H]

FFS! 10 seconds is trivial!

Way back, I had a little ¾- Watt VFO exciter (with a tone generator so it was easy to find on the band) that matched into 50Ω pretty accurately. I built it into an SWR bridge case, and used it for years to calibrate aerials. A "beep" on Band II tends to be ignored by listeners, so it was unlikely to be noticed. I used to set it to the frequency that I wanted to match to, and tweak the antenna for best SWR. It used to take about 5 minutes to perfectly tune a Band II aerial for best match!. It was battery powered and cost almost nothing to build. The output stage was a pair of 2N3866s in push-pull, into a matching transformer for exactly 50Ω match. I think that I might have the thing around here somewhere, still!

[piratefm]

FFS! 10 seconds is trivial!

Way back, I had a little ¾- Watt VFO exciter (with a tone generator so it was easy to find on the band) that matched into 50Ω pretty accurately. I built it into an SWR bridge case, and used it for years to calibrate aerials. A "beep" on Band II tends to be ignored by listeners, so it was unlikely to be noticed. I used to set it to the frequency that I wanted to match to, and tweak the antenna for best SWR. It used to take about 5 minutes to perfectly tune a Band II aerial for best match!. It was battery powered and cost almost nothing to build. The output stage was a pair of 2N3866s in push-pull, into a matching transformer for exactly 50Ω match. I think that I might have the thing around here somewhere, still!

If you find it, I’d buy it! Hahaha