7.5dBd gain on a vertical 5/8.
Is it really this ???

> Hi,
> Have you ever done any analysis on the Sirio 827.
> I have this antenna and the new 5/8 Antron (IMAX 2000).
> I am wondering how Sirio can quote a 7.5dBd gain on a vertical 5/8. Is it really this ???
> You don't need to publish the results, I'm just curious to know.
> If not, could you give me some links.
> Cheers. Rob, 163 DS 92

Hi Rob.

When I received your message, a few days ago, I had two choices, the first one was to answer -useless claims, as usual- but the second one was to further investigate, so because I'm now  incurable ...

But we must start far back where theory assign gain to (thin) dipoles in free space.


Theory,  dBi

Computer,  dBi

1/2 lambda 2.14 2.17
1 lambda 3.73 3.91
10/8 lambda 5.18 5.10
3/2 lambda 3.44 3.57
2 lambda 3.96 4.04
5/2 lambda 4.78 4.90

The 10/8 dipole offers the max gain. So the corresponding vertical, the 5/8, is well known as the best vertical antenna.

The theoretical gain for common vertical monopoles, of course over perfect ground, would be:

1/4 lambda

5.18 dBi

1/2 lambda

6.92 dBi

5/8 lambda

8.11 dBi

frspace.gif (6732 bytes)

So when a vendor claims 8 dBi for a 5/8 GP he's maybe claiming that -over perfect ground- gain. This is not so unfair when we know what we're talking about. When a vendor claims 7.5 dBd, this is 9.64 dBi ... that's far over the theoretical gain.

I don't have the actual sizes for Sirio 827 so I can't model it. But that antenna it's a clone of the older Sigma Mantova Turbo, which was the first antenna on my roof ;-) and trust in me, all 5/8 must have similar gain when decently built because an aerial has a max teoretical gain that can't be exceeded but only worsened. 

So I will use the Mantova Turbo as an example.

I tried again and again to force the computer model for the max gain. I set unreliable parameters as perfect ground and no-losses materials and the max reported gain for the Mantova was 8.46 dBi with the antenna directly connected to perfect ground, read with no radials. With the 8 radials I was forced to raise the antenna at 10 cm, hi, over perfect ground and the software reported 8.27 dBi.

As of course perfect ground don't exist in the real world, I tried to extract the max computable gain over salt water too and found it was 7.06 dBi, 17m. (1.5 wl) high over salt water, but up at an undesirable vertical angle of radiation, while dx performance would be better with the antenna low at 1 meter high or below.

saltwat.gif (7354 bytes)


What about real ground? Here the higher the better is the rule and, after ton of tries, the max registered gain was 7.44 dBi up at 817 (sic!) meters, and with a very low vertical angle of ... 0 degrees ... but of course you wouldn't to raise it sooo high, due to cable losses, right? ;-)

On the last, what can we expect by a 5/8 GP in the real world? If you have salt water or exceptional ground conditions, a vertical will be great for dx. If not, when you can put up just a simple horizontal  dipole at 11m, that dipole will defeat  the 5/8 vertical. But at 5m high both aerials will perform almost the same at low vertical angle, and below the 5m high the vertical is for sure a better choice.

realgrd.gif (6577 bytes)

Hi. 1 AT 1839 Paolo