Modified Vector 4000

[Shockwave]

However, since the currents on either side of the feedpoint are not balanced, a choke balun is mandatory to suppress unwanted currents on the feedline"

That statement seems to be in alignment with what I've found, having to tune the cone to 1/4 wavelength in order to remove CMC problems. CMC does increase when the cone deviates from 1/4 wavelength.

 

[Marconi]

Bob, I also never got around to tapering the radials either. That looks to be big chore.

 

[bob85]

It looks like you have scaled tube diameter Eddie,

I only scaled for length on 50mhz & hoop diameter to get the radials close to the same angle & spacing relative to wavelength,

relative to wavelength I am using a fatter monopole & radials than any of the 27mhz versions,

bottom tubes inside the cone are 37mm od & 33mm od, 5 sections in total,
radials are 13mm x 1400mm & 10mm x 1300mm,

I can measure the exposed lengths & diameters tomorrow but not the top section until they are tuned.

 

[Marconi]

That statement seems to be in alignment with what I've found, having to tune the cone to 1/4 wavelength in order to remove CMC problems. CMC does increase when the cone deviates from 1/4 wavelength.

I believe Donald is right on this idea. My S4 with a physical gamma and with shorter radials than the NV4K requires a chocking device to mitigate the CMC for the model. The difference in the tabular currents file shows very little difference between these two models, S4 with and without a choke, but the gain is noted to improve as noted in the far field patterns below.

Donald must have had his Field Strength meter fine tuned and ready to go, on the day he discovered this tid-bit of information, in order to see this little difference.

 

[Shockwave]

I believe Donald is right on this idea. My S4 with a physical gamma and with shorter radials than the NV4K requires a chocking device to mitigate the CMC for the model. The difference in the tabular currents file shows very little difference between these two models, S4 with and without a choke, but the gain is noted to improve as noted in the far field patterns below.

Donald must have had his Field Strength meter fine tuned and ready to go, on the day he discovered this tid-bit of information, in order to see this little difference.

In my case, not having the cone covering the first 1/4 wave of the monopole inside, could create enough CMC that RFI would sometimes get into the audio rack. On FM, this RFI feedback sound like a hum. At 1/4 wavelength, this problem goes away.

Now, the need for a choke also explains why it may have been easy to miss any gain increase while experimenting without a choke or mast isolation. I am curious to see how this turns out.

 

[Marconi]

It looks like you have scaled tube diameter Eddie,

You're right Bob, this is not what I intended to do, but it is what happened with the wire diameters in my post above.

I was intending to compare my tube lengths with your new 50 mhz antenna, hoping to see how close I was to your model. In the process of trying to get the wire diameters to stay the same as I had at 27.205 MHz and when I scaled to 50 MHz, I mucked-up the presentation without realizing it. I sent you some bad wire diameters.

I think I have the necessary wire diameters for the Vortex M2, except maybe to the top 3 or 4 wires for the radiator and I think I can scale those and maybe get close enough.

I was dreading making the radials with taper and I not have good dimensions for their lengths and not know if the radials are 2 or 3 wire didn't help. They for sure will remain as a single wire about 161" inches long at 27.205 MHz in my case. I have two goals now.

1. to see if Vortex is puffing about the gain they report.
2. to try and make a model like your antenna, currently waiting to test, just in case you're right and that without the exact and correct dimensions my model of the Vortex is not likely to be correct.

I realize you are using some Vector parts for your Vortex test, and I have figured out what I think are good dimensions, except possibly for the NF4K offset for the radial hub. My question is...do you have any idea what radial L bracket dimensions are on the Vortex? I use this dimension L" to help me tune the Vector model, and I can't be sure that is a good idea. If I used whatever it correct at the point on the antenna, and I did not get some good results...I would have to rethink my modeling.

To be frank Bob, I think a modeler that has a few minor errors hidden in his model, so-to-speak, can coverup such errors in the process of tuning the gamma match design I use in these models. Isn't that what you tend to see with your real antennas with a matching device?

BTW and just as a note, I notice that your gamma on both antennas looks approximately the same. In my earlier error prone models of the Q82 M2, I found I had to set the dog bone tap point at 60" inches above the feed point vs. the NV4K set at 32.5" inches.

For sure that could have all been due to a dimensions error at the bottom tube for the antenna however, and also considering I was estimating the average radiator diameter as 1 wire, top to bottom, set at (0.73" - 0.84" diameter) instead of using the correct taper and various diameters per tube.

How are things looking for your testing?

 

[bob85]

Eddie,
Vortex used custom made tube with oddball unequal taper steps,

http://www.vortexantennas.co.uk/shop/category/accessories/aluminium-tubing-6082-t6-or-aa6082-t6/,

I don't know the offset at the base of the cone, but looking at oggy's video the radials are 20mm so the offset is at least 25mm maybe 30mm,

I have not done any tuning & testing today so nothing to report at the moment.

 

[freecell]

Marconi wrote: "when I scan the SWR the antenna shows resonance at 44.8 MHz and IMO that is not right either."

the radiator is too long for resonance @ 50-54 mhz.. shorten the radiator until reactance equals 0 at the preferred frequency. check the match and readjust the gamma as necessary, check reactance and readjust for X=0 if necessary. repeat procedure until R=50 jX=0 at the preferred test frequency. once X=0 the impedance will present a non-reactive, purely resistive 50 ohm load.

 

[Marconi]

I don't know the offset at the base of the cone, but looking at oggy's video the radials are 20mm so the offset is at least 25mm maybe 30mm,

Bob, just looking at the base of the Vortex Q82 Mark2 in Dave's videos, below, at about 14 seconds I can easily see this off-set dimension could be close to 3" per radial at 27.5 MHz due to the large mounting brackets noted in the image below. Also note how wide the base looks including the radials.

www.youtube.com/watch?v=Uq0ulNbKlWo&t=95s

My current model at 27.5 MHz the has the off-set at 3.5" from the center of the base. If I ever get this model right and it shows the fantastic gain reported...I can only imagine it will be similar to what I think you suggested earlier, "...that I would have to get the dimensions right in order to see the real results."

 

[bob85]

look at part 2 of the video Eddie, you get a good look at the radials & brackets,

if you mean the length of the bracket bolted to the side of the 2" box section its about 25-30mm to the bolt hole,

fatter tubes need more spacing skinny tubes need closer spacing,

imho getting it a bit wrong will effect the resonant frequency a little but the gamma will give a good match.

 

[Marconi]

Marconi wrote: "when I scan the SWR the antenna shows resonance at 44.8 MHz and IMO that is not right either."

the radiator is too long for resonance @ 50-54 mhz.. shorten the radiator until reactance equals 0 at the preferred frequency. check the match and readjust the gamma as necessary, check reactance and readjust for X=0 if necessary. repeat procedure until R=50 jX=0 at the preferred test frequency. once X=0 the impedance will present a non-reactive, purely resistive 50 ohm load.

FC, I had something wrong in the model and I made a few changes. Not sure what made the difference, but I got the model to work with the radiator length remaining at 3/4 wavelength and the radials at 3/8 wavelength.

Since we only knew the length of the radiator and the radials, base on a % of wavelength my goal was to model without changing the design elements lengths and diameters if I had the details...and use the matching device for its intended purpose if possible.

With that said, in the model below I still have some mistakes in making the radiator 3/4 and the radials 3/8 wavelength at 27.500 Mhz. I also have not attempted to set the model using taper. So, in this case, as with my earlier models of the Q82 Mark 2, my tuning the gamma match covered up my mistake. I'll fix that eventually, and we'll see if it makes a difference from the reported gain below and the Vortex reported gain of 5.82 dbi gain.

And, the gain still sucks!

You will also see in the several antenna view images I posted below that I have started to use taper, but all I can do for now is to guess at the radiator and radials lengths. Sometimes it takes a long time to flesh-out secret information such as antenna dimensions. The model below is just another model in progress.

 

[bob85]

I may be inclined to agree Eddie,

although im not done tuning the 3/8wave i have something to add confusion to the mix that im sure getting the vswr down lower than 1.3:1 on the 3/8 won't fix,

i tuned the 1/4wave version today no problem with the typical vswr curve & 1.02:1 match,

the 3/8 version does have a much wider vswr bandwidth but its not as easy to tune imho
i have had 3 different gamma tubes & rod configurations on it & its still not flat like the 1/4wave version,

i did a quick test against the 1/4wave version then talked with Nav for a while before repeating the test to make sure im not tripping,

first time around we tested on different days because i modded the antenna to 3/8wave from 1/4 cone & properly cleaned it up,

this time its a straight swap in short time on the same pole & place as the first test we did,

we had 3.6dBm difference between the two in both tests we did tonight,

but this time the 1/4wave cone has the advantage.

maybe when i test them higher & isolated from the mast & coax the 3/8 will shine but i doubt it,
Their tests were not done isolated,


The 1/4wave cone version feels right when you tune it,

i can't say that about the 3/8wave cone, it does tune & if all you use is a vswr meter it looks low & wide.

more tuning & testing tomorrow if the weather holds off.

 

[Marconi]

Bob, when I finally made a Vortex model with 3/8 wave radials and using a physical gamma match...I had to make the gamma match dog bone spacing 60" inches long, from the bottom of the radiator wire to the dog bone...before it would match well. I also thought I saw a larger than normal gamma length in Dave's video too.

I may have already said this before - somewhere in this thread. But, I wasn't real sure, because this was just a model using Eznec.

 

[bob85]

i have had the gamma strap over a wide range of settings with different gamma tubes & rods Eddie,

weather permitting i will carry on tomorrow & see if i can get it tuned as low as the 1/4wave version,

ignore the first test done on different days,

tonight we found signals change over time, even though we had the same 3.6dBm difference between antennas in both tests the two tests gave different signals,

for me & Nav swapping antennas fast is important, whatever is changing signal strength with time is likely screwing the whole test up so i swap as fast as i can,
its all i can do at the moment in lockdown.

 

[Shockwave]

tonight we found signals change over time, even though we had the same 3.6dBm difference between antennas in both tests the two tests gave different signals,

for me & Nav swapping antennas fast is important, whatever is changing signal strength with time is likely screwing the whole test up so i swap as fast as i can,
its all i can do at the moment in lockdown.

This is definitely not an issue only experienced in yours and Nav's testing. Signal change due to propagation changes is a major problem when measuring gain down to something as little as one db, over a distance that could be considered usable for optimizing antenna gain on the horizon.

This type of signal change over time, with no changes being made to the antenna under test, have messed up more than one of my results. The expanded loop 11 meter version built many years ago was a classic example of this mistake that only become apparent after several days of having the new antenna installed.

The only way I've been able to feel confident that this problem was not effecting my test results, was having the ability to repeat the same test, at different times, with the same results.