New Sirio 1/2 wave Gain Master

[Marconi]

I'm not even sure what model big sticks we had here,
The antenna we called shaki big stick in the 70's had a coax tail hanging out of the bottom, it was shorter and thinner than what we called the super big stick,

As I understand you description I had one of those I think. Long ago it was described to me as a no ground Marine Big Stick made by Shakespere. It was a 1/4 wave radiator with a flexable coaxial 1/4 wave return current element like you described.

my super big stick looked like the 176 with a rubber boot over the joint,

I don't know what a 176 is, but I've had a couple of 2 element big sticks that used a rubber boot in the middle. The antenna worked, but I did not like the performance of any of these antennas.

 

[bob85]

Mine was the two piece with rubber boot eddie, I never liked the performance either,

In multiple tests against other 1/2waves they were the worst performers of any i owned, maybe they needed decoupling from the mast and a choke,

They were strong in bad weather, i only ever saw one that broke in the wind & that had been up for years on a short rigid pole, gainmasters are nothing like as strong.

 

[Marconi]

They were strong in bad weather, i only ever saw one that broke in the wind & that had been up for years on a short rigid pole, gainmasters are nothing like as strong.

I don't have steady winds like you do, but I've had my GM up since sometime in December of 2010. Several months ago lightening hit a nearby tree and took the antenna out...likely with a spike. I checked all of the joints and they were fine, no splits or signs of wear.

If I can find my pictures I will post the damage. I'm surprised that I have anything left at all...based on other lightening strikes I've seen. I kept saying I was going to have it taken down, but I waited too long.

 

[The DB]

can you insert a capacitor of different values into the model to see what effect it has on the match?
i have seen one version of j-pole with a series capacitor to brodband the antenna,

Doing some modeling. Here is the SWR curve for a center fed T2LT in free space.

You will note that the SWR curve is set to the same scale for SWR and frequency as it is in the linked manual above, and was added here for comparison purposes.

Mounting this T2LT model 1/2 wavelength over moderate earth has a very minor change in said curve, so minor in fact that you have to look at them back to back to see it.

If I put the equivalent capacitance of 1 inch of LMR-400 coax (2 picofarads) just above the feed point, the SWR, and resonant points are completely changed, and drastically, as is the current distribution. Adding in that small amount of capacitance destroys anything resembling a match putting the SWR of the antenna in the multi thousand range.

A look at the version with said cap's current distribution.

Changing the length meters at a time does not even come close to fixing the reactance and SWR problems that adding a capacitor in that location causes. Because of this, I have to conclude that if there is a capacitor in the HW Gainmaster, the feed point must not be in the exact center.

Further, I think adding such a capacitor anywhere in the system will make it impossible to keep the RF current pattern that the documentation claims.

Another thing I will point out is one thing quoted directly from their documentation...

The design of the radiant element works like a central feeded dipole

At this point I ran some checks on my original vertical center fed dipole. AGT came out at 1, which is a perfect score. I made the elements copper, and put 2 millimeters of fiberglass around said elements. These changes had very little effect on the SWR curve shown above, again, so small of a difference that if you didn't look at the two curves back to back you would never see said difference.

Unless someone has a suggestion to try, I think I am done with this analysis.


The DB

EDIT: I just had a thought. The antenna's feed point for the model is not the same as the antennas feed point that you plug the coax in to. I wonder if the type and length of the coax they use would, at least partially, be responsible for the difference between my SWR curve and theirs, which really aren't all that different to begin with. Coax will have the effect of lowering SWR across the bandwidth of the antenna, thus giving it a lower SWR low point, and a wider bandwidth as visible from the point that you attach your coax...

 

[bob85]

Holy cow, never seen anything like that happen round here, Im very happy we don't get lightening like that.

 

[bob85]

That's interesting DB, 2pf is around 2900 ohms reactance,

since current inside the coax is always equal magnitude & opposite phase,
is the model showing us that voltage on one side of the coax capacitor is rising to very high levels until equal current flows on the capacitor as flows on the outside shield of the lower 1/4wave?

what about a much larger capacitor, say something closer to 2000pf a few ohms reactance, does that bring current above the capacitor back towards what is flowing on the lower 1/4wave,

Thanks

 

[bob85]

Here is the article using series capacitors to increase vswr bandwidth of antennas
https://www.antennex.com/preview/Sep09/the_x-pole.pdf

 

[The DB]

That's interesting DB, 2pf is around 2900 ohms reactance,

since current inside the coax is always equal magnitude & opposite phase,
is the model showing us that voltage on one side of the coax capacitor is rising to very high levels until equal current flows on the capacitor as flows on the outside shield of the lower 1/4wave?

what about a much larger capacitor, say something closer to 2000pf a few ohms reactance, does that bring current above the capacitor back towards what is flowing on the lower 1/4wave,

Thanks

Yep, capacitance is inversely related to the impedance the capacitance creates. 2000 pF and the current distribution looks almost like nothing happened. The problem is if they are using coax, there isn't enough room in said coax to get anywhere near that much capacitance.

Now that I go back and double check, that capacitance value, which was entered in farads, not pico-farads, which causes me to need many leading 0's, and my data had one to many 0's in it. Apparently 0.000000000002 and 0.0000000000002 makes quite a big difference...

With that change, the current distribution on one side of the feedpoint is about 50% larger than the other side. SWR and impedance is still way off, being calculated at 1528:1. That capacitance figure is an underestimate as there is coax that has more capacitance per foot than the LMR-400 I got that data from, such as RG-8x... The area in the image also looks like it is longer than the 1 inch I used...

I used three inches of Times Microwave RG-8X this time, and I used the correct number of leading 0's. Using said capacitance, and with adjusting where on the antenna the feedpoint was, and how long the antenna was, I was able to get SWR down to 4.89:1. This when the antenna was 7.4 meters long, or about 24 feet. This is way beyond a half wavelength antenna,and is in fact closer to a 5/8 wavelength antenna... Like the original Gainmaster.

For fun, I added a stub match, like the original Gainmaster has, and then messed with its values and came up with this SWR curve...

That is almost 5 MHz of SWR bandwidth... That is awfully Gainmaster like don't you think? That is in an antenna that is 5/8 wavelengths in overall length for 11 meters... This antenna also passes the AGT test with a 1, or a perfect AGT. This I think this is worthy of more experimenting...


The DB

 

[bob85]

Yes the vswr curve of your 24ft model looks gainmaster like in shape, it's also shaped like the curve in the extended & maximum bandwidth versions of the Xpole,

I suggested 2000pf out of curiosity to see how that would be displayed in your software as it's not clear to my untrained eyes where the current leaving the centre conductor that causes that magnitude of return current on the braid is been displayed,

thanks.

 

[The DB]

That was with 7.7 pF, which was based on the capacitance of three inches of RG-8X coax.

Unfortunately I edited that model that produced the above SWR curve by mistake. When I tried to recreate it I got close, but not quite as wide of an SWR bandwidth on a different curve... Also the charts that show X and R over the same frequency range.

I really wanted to include a smith chart that shows the frequency sweep in this bandwidth as well, but I think it will likely just confuse a lot of people. However, if you are familiar with smith charts I think it would explain why this wide bandwidth happens with this type of antenna... If someone is interested in seeing it, let me know.


The DB

 

[bob85]

If you can give us some explanation it would be interesting DB,
i would also like to know if these wideband antennas have decent gain across their bandwidth or do they only perform well over a narrow portion of the wide vswr bandwidth.

 

[The DB]

If you can give us some explanation it would be interesting DB,
i would also like to know if these wideband antennas have decent gain across their bandwidth or do they only perform well over a narrow portion of the wide vswr bandwidth.

I'm happy to post the smith chart if someone wants, but I'm not going to give an explanation on how to read one right now. Explaining what one sees on the smith chart will take some thinking on my part. Its not a real complicated device, it looks far more complex than it really is, but I'm not prepared to explain a smith chart right now, and I haven't done it before.

When it comes to gain, I'll post that here. I bet it will be different than you expect...

The gain falls right in with what I expect to see with such an antenna with the lower frequencies having less gain than the higher frequencies, this makes sense if you think about it. Also remember, this is frees pace gain. The angle of maximum radiation in free space all through the frequency range is 0 degrees. Being free space gain, you can directly compare it to a dBd reference if you like. There is no earth involved here screwing up the comparison...

Also, don't mind the Fr/back chart att he bottom. That is there because of the stub that 4NEC2 used as the matching system for the antenna, and it is extremely small to the point of being irrelevant...


The DB

 

[Marconi]

Yes the vswr curve of your 24ft model looks gainmaster like in shape, it's also shaped like the curve in the extended & maximum bandwidth versions of the Xpole,

DB, I don't understand any of the matching and design modeling you have posted here. I also missed seeing where you made your T2LT model 24' feet long like Bob suggested.

IMO, this longer radiator is probably what is producing the SWR bandwidth curve that looks similar to the Gain Master. Click here: http://hittman.us/pictures/3-9-16/testswr.jpg
Many of us have likely seen this BW curve shape somewhere on the Sirio Website for their original Gain Master.

If I made my New HF Gain Master 24' feet long instead it being a 1/2 wave at about 17' feet...it would produce a similar antenna pattern to the GM too, and my model does not include the matching device you're showing us.

Your model also shows us a unusual image of the current distribution. Click here: http://hittman.us/pictures/3-9-16/dist.jpg
and I'm guessing this is simply due to your adding the matching device. I've made models before that showed similar current distributions and I found out later the model was broken.

Try removing the device temporally and see it the current distribution pattern doesn't look more normal like a GM. You won't have a good match to produce a nice SWR curve, but I don't think that makes much difference in the performance results.

I know you will probably say that I don't have a clue as to what you are trying to do, and you are correct, but I don't think a matching device is going to broadband this antenna appreciably without physically making the radiator longer.

Just my opinions.

 

[Marconi]

Also, don't mind the Fr/back chart att he bottom. That is there because of the stub that 4NEC2 used as the matching system for the antenna, and it is extremely small to the point of being irrelevant...

DB, Eznec warns us in the Manual...that if an object is irrelevant, it can and should be removed.

 

[Marconi]

Bob, I will be very surprised if a simple 1/2 wave radiator can be broadbanded much more than it is. I also think if it was possible the guys at Sirio would know it and make it so.

Just my opinion.