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Discussion in 'CB Antennas' started by kbbgood, Apr 22, 2013.
Thanks for the info Troubleshooter.
Good luck with the new antenna.
Well IMO, Eddie, isolating it will better show the effect of the radials, but one thing I noticed in your last model of the IMAX at 41' (Thank you!) was the current bubbles looked different than The DB's did in that his seemed to have the larger bubble on the bottom whereas yours appears to be bigger on top, which has me wondering if you placed the cap possibly higher than it is in The DB's models?
Seems he used segments 31 & 32 of 64 for the cap location, but apparently he's beginning at the radials and counting up whereas possibly you're beginning at the top and counting down?
...and did anyone ever decide if the cap was 25 or 42 picos?
The cap is 42, again it was a mistake. I make them all the time, but I try and clean em' up too...when I find out.
I will consider your comments about the radials and the bubbles, but I will talk about the segments 31 & 32 of 64 for the cap locations.
What is the middle of 64, about 32? If I placed my capacitor at the middle and at segments 31 & 32 of a 64 segment of this long radiator, and I counted down from the top, and then suppose DB did the same but measure from the bottom...then we should still be close and show similar results...is my opinion on the matter. Then the only answer to the difference you see is because he probably used a much longer radiator that I did maybe. I don't know, but I have asked him what radiator length he used for his Imax model. One day he might tell me, but maybe by then I will have forgotten about the whole matter.
Maybe he doesn't want to argue with me about his using one length and me another, saying it does not matter.
I used an overall length of the physical radiator at 268.5" inches. I think that is what the physical length of the Imax radiating element actually is when measured.
I've explained this already right here in this thread, but I know it can be difficult sometimes to keep up, with everyone being busy and popping now and then.
I did finally try to duplicate the model that DB called his reference model and I had to lengthen my radiator out to maybe 284.5" inches just trying to get similar results.
This may be why you see a difference, and I'm just assuming we are looking at your facts and they are the same facts. I have models laying around all over my desk that I'm checking out, and I have a file with hundreds of models, some I've posted some I haven't, but I just don't try and remember much anymore.
So, please help me out a little. I can't read minds either.
NB, when you look at an image and you want to use it as a reference, why don't you consider to at least get the full title or a link would be much better. This will help me understand what you are talking about without my trying to remember.
When you get to a post that has the reference you want to use...just click on the post # at the bottom right of the specific post and a window will open. The select and copy the bottom link, then paste that copy to where ever you want, and wa-la even the guys in Rio-Linda will know what it is you're referring too. They can see it and reference it, and refresh their recollections, and then maybe even talk to you about it.
"Rio-Linda" - LMAO, I used to live 15 miles from there, and so did Rush, I believe!
OK on posting or referencing the specific examples, (and I believe I had those wack-birds anyway) your current node (bubble) was larger on BOTTOM, DB's on TOP.
So, you're working with 64 segments total, and 2 are used up by a capacitor, leaving 62 for the radiator.
If the 32 on top are unadulterated then you have only 30 on the bottom with a capacitor equal to 2, 31 & 32, connecting them in /near the center.
In that case the top is going to be about 7% longer.
You'd have to add it in using segments 32 & 33 to center it, which makes me wonder exactly where in the total length of the Imax wire between the matching circuit and the tip, the cap is actually located, and what amount of difference it makes?
- Also, I noticed a small out-of-phase current at the base of The DB's example model, and wonder if minimally changing the location &/or value of the capacitor might eliminate that and instead we might see more symmetric top & bottom currents...???
oops, it pays to read ALL the posts
NB, I've already answered what I thought might explain why DB's model and my model didn't look alike. Here is the link to my previous explanation to what I thought might be the cause of the difference.
Best groundplane ???
Below is my Eznec models where I tried different model assumptions to try and duplicate what DB did to start with. I noticed the difference in length as noted in the two examples in the PDF file below. Check out the antenna details in the data block at the lower left. Like I said earlier, our differences in lengths for this radiator is not the point...but it will make a difference in results.
So, maybe he doesn't want to argue with me the actual radiator length for the Imax. I don't want the argument either but we did use different lengths, and again it does make a difference to what you see...and I might add it makes other important detail differences in results as well.
This is why our patterns we each reported do not look at least close to similar.
Check the PDF file below.
Well, the Imax is 24' total. Remove 12" for the base bracket which simply has coax inside and now you're left with 23'.
Subtract the length of the matching network, what, maybe 6"?
You'd be left with 22' 6" or 270", minus, what, maybe 1.5" of capacitor?
Now you'd have 22' 4.5" of radiator, however, what about the VF? (Velocity Factor)
Inside fiberglass isn't going to be the same as air so it's bound to be slowed a bit, so what, maybe 99% VF?
268.5 ÷ .99 = 271.25"
SO, Let's see what that would leave you for frequency if it were a 5/8...
271.25" ÷ .625 = 434"
11803 (speed of light in million of inches/sec) ÷ 434" = 27.195MHz
I could live with a "one-channel-off-from-perfect" antenna length - for the sake of calculation, anyway
But I'd use 271.36" for my modeling, if you expect the Imax was designed to be a "resonant 5/8" at 27.185MHz.
NB, my model of the Imax in this case does have a capacitor added, but that was done in a model by DB to show the effects. I think DB modeled the Imax with a capacitor because someone reported that his Imax showed no continuity in the middle section of his Imax and he was just curious I guess.
I tried to duplicate his work so I could try and understand this idea, but I saw different result than he did. I believe it is due to the difference in radiator lengths we used. So I agree you dimension for the Imax is very close to what I get, and somewhere I explained how I determined the overall length that I got.
My model does not match even with the capacitor added, so there is no way I can check it for resonance.
You will have to talk to The DB if you are interested in seeing what length radiator is resonant in the Imax...I think he can model a match necessary to fix the Imax with the capacitor included...so it might show us resonance if he will follow up.
As a note however, I find that within reason, modeling an antenna with a good match or bad...does not ill-effect the performance values like gain and angle. It does allow for good numbers for things like the SWR, the feed point match, and bandwidth, however.
So, you can encourage him to get crackin' on the idea...maybe he can match the Imax and tell us what length gives him resonance in the middle of the CB band. I for one would like to see that happen with the capacitor included.
I can't explain the really nice gain I see with my model simply adding this capacitor, and I would like to know more.
I have noticed the SPT 500 is not as broad banded as some of the others it is more geared for 11 Meters
I've used many A99's (with and without ground planes), at many different heights over many different decades (over a 40 year span) and the P-500 is quite noticeably better all the way around. I have also used a Maco 103C (Vertical) and that was quite the performer. No comparison when it comes to reaching out, finding and keeping a contact, even if that contact is moving at 70MPH down the interstate. (Something the P-500 could never do at my location). But, if you have land 360 degrees in all directions with CB'rs scattered with-in a 35 mile circle surrounding your antenna, you will not find a noticeably better performer than the P-500, unless you go with a beam. If I were to set up another station for a friend or relative, the HyGain Penetrator would be at the top of my list. Never tried the Gainmaster and have never talked on the air with anyone that had one, so my bias is going to be with what I know to be a great performer.
Ferrarimx5, I tend to agree with you, but thus far my Eznec models do not prove for me that the SP500 is any better than any of my longer high gain CB antennas at the same mounting height.
I'm still holding out hope that my current experiments with this idea will show some positive gain results. I'm trying to get the physical matching device to work right after adding HG's raised radial mount design.
I'm close but right now I don't have good control over resonance base on my use of the sketchy dimensions I have for the SP500 beta type tuner. Getting the dimensions right in this area on the antenna is difficult.
Right now I'm waiting on Needle Bender to get through with his move to a new home...and he will get back with me regarding the original wire dimensions for the 23 channel model according to him.
I have some dimensions for the New SP500 matching device, but they are incomplete too.
Which SP500 are you working? If it is the new model...maybe you can give me some assistance with your dimensions, OK?
I'll try and keep you posted on my progress.
The PDF image below is my model. You will note that the matching device dimensions I have here are not working for the middle of the CB band where I believe this antenna was designed.
After adding the matcher...the resistive part of the impedance match was pretty good, but it was far from resonance. I adjusted the resonance by changing the length until I saw near X=0.00 ohms. On doing an SWR scan the resonance showed to be at 29 mhz, and the radiator was way long. Still in error, but the device did work to fix the mismatch. This matcher must work really hard to tune the SP500.
The advantage I find possible with this matcher is it adds a very strong horizontal RF field to the radiated pattern as noted in the far field pattern attached below. The pattern is noted to be irregular, but I think we might expect to see that with the shape of the mount and the addition of the matching wires at the base...where there is much current activity going on.
The big thing here is the gain is strong and if corrected matching can result in little to no change in the pattern...we may understand a little more about why this antenna has received rave reviews for years...with very little descent I might add.
If you have any information on the NSP500 matching device dimensions...let me know, OK?
NB, your dimensions for 65" radials do look to show a bit more gain.
The model I did above has 108" inch radials. I think all of my models with shorter radials for the Imax always show better gain that using 108" inch radials. Go figure!
How are you doing with your move bud...I need those dimensions for the SP500 you promised?
Marconi, without pulling the antenna down and measuring, my best recollection is each of the four radials are 105" and the total OAL of the antenna is 272 inches.. I did tune the antenna with an MFJ-259B for 27.225, so there is probably an inch of play to the OAL, but this is what I remember for the initial set-up (Before tuning). When I build the Maco 103C next week, I will measure the P-500.
The MACO is coming from Michigan and have not received any shipping details or tracking number. PalcoElectronics (Copper Electronics was closed at 5:00 pm, so I ordered from Palco).
Marconi, you are a real asset to this community.
Thank you for all that you do!
Yes he is an asset!! Helps prettty well anybody out when they need it and are willing to work with him on the measurements. You'd be suprised at what Ol' Grandpa can do!!.
Ferrarmx5, take your time putting the Maco M103. I would get some blue loctite and use it on every bolt, nut, and screw. Also using NO-OX special "A" is a good robins everywhere except the feedpoint connector.
And make sure to weather proof it good as well. I like self fusing or what is really called self amalgamating tape. It works very well on connectors as it won't stick to them, only to itself. And it does a good job of weather proofing if put on correctly. And also make sure you have a good ground system. All your equipment as well as your antenna mast and ground rods should all tie together to form a single point ground system. And last but not least, a good surge arrester. Polyphaser, Huber Suhner, Andrew, Commscope all make good ones. There are other companies too, these are just the ones I can think of off the top of my head.
Anyways. I am sure you know all this stuff, but sometimes it never hurts to make sure. And make sure to either take readings directly at the antenna feedpoint with your antenna analyzer or use an electrically tuned 1/2 wave coax jumper cut for the frequency you desire. I use the following formula. 492 * (VF of coax being used) / frequency. Like the following. 492 * .83 / 27.400= 14.903ft. This would be a 1/2 wave length electrically tuned coax. You can then use multiples of this number for a longer coax if you desire. And yes it works. I just did this and tuned my antenna perfectly.
I can now use random lengths of coax and not affect the tuning of the antenna at the radio. It's the as the feedpoint and using the 1/2 wave coax jumper. I used a 71ft coax jumper and it gave the same results as the other methods that were used, directly at the feedpoint and also a 1/2 wave electrically tuned coax jumper cut for 27.400MHz. Just some food for thought and be safe installing that antenna. Wish I could get something like that going without my neighbors hating me LOL!! Houses are a bit too close here. Anyhow. Again, be safe and good day.
73 and God Bless.
The 272" and 105" is about right for the radiator and radials and I have that. What I'm trying to do is add the physical matching device to the model. I have two partial dimensions take-offs for the matching wires, but neither works as yet. Both are missing dimensions and then I have to just take a wild guess.
Thanks for the offer to get me your dimensions of the SP500. I prefer to get the New SP500 model dimensions, but if you can help give me whatever version of the SP500 you have will be fine. I just want to find some good data that works first.
I have a model that I refer to as my original 23 channel version, but I'm not sure which antenna my buddy measured for me. Problem is, it does not work to tune in 11 meters. So, I have errors or bad dimensions somewhere or both.
I really want to model the New SP500 which has changed the matching design a bit from the older versions. The ground wire is on the opposite side of the antenna too. But, any version that works is good and will be greatly appreciated.
Here is a good example of what one of my buddies sent me several years ago. I believe his dimensions were a take-off of the original 23 channel version, but that is just a guess...I never questioned the dimensions and I did not know then there were different matching designs out there...except for the length.
The PDF file below will give you an idea of the type of detail needed. Every little length of these wires must be accurate and then input to the Eznec wire editor correctly, including length, diameter, and height. Note: some wires add to the height of the matching device and some do not.
In other words, there is a difference between the overall wire height and the overall wire length and understanding and getting that right is essential to modeling just like it is on the production floor of the manufacturer.
Check out the ground wire for the SP500. It shows to be 10.5" tall, but the total over length is roughly 14" inches long. The bends are important and in this case they are 90* degree bends and add nothing to the height. But, if they were bent at an angle then the height would be a different overall height...if you get my point. Back when I got these dimensions I did not consider the angles and the difference in length and height and it can make a difference.
I hope this makes sense Ferrarimx5 and thanks for the kind words.
See an example of incomplete dimension take-off below.