mobile antenna to base !

[Riverman]

I'll buy that.
Now, how efficient or inefficient is the antenna with its 72 ohm feedpoint impedance if no balun is used insuring a good match with the 50 ohm coax? Is it worth worrying about?

 

[Marconi]

I'll buy that.
Now, how efficient or inefficient is the antenna with its 72 ohm feedpoint impedance if no balun is used insuring a good match with the 50 ohm coax? Is it worth worrying about?

Well 71, I figure you have heard plenty of guys say that any match less that 2.00:1 SWR is safe to run, but I'm not sure how efficient the idea is. IMO there will be some CMC's on the feed line.

Maybe the model with a mast I added below will give you an idea of the difference, in running or not, a device is to help eliminate the issue.

This model does not have a feed line, but the mast acts similar even though its diameter is larger. The red line is the currents predicted by Eznec for this setup at 36' feet in the clear.


Here is an overlap of these models, with and without the mast included. Does this look to make the kind of problem you were thinking?

That said, depending on location and installation...you mileage could be different.

Good luck and keep us posted,

 

[Riverman]

Am not too concerned about rf on the feedline. Have never noticed any ill effects on any dipoles I've ever used whether they were made with wire or mobile antennas.

Instead was wondering about how much the wattage out the antenna is reduced due to the inefficiency caused by not having a better impedance match. Does that make sense?

 

[Marconi]

In the models I posted here the answer is NO! and the models show that.

 

[Riverman]

In the models I posted here the answer is NO! and the models show that.

Are you saying the wattage is not reduced or that my question doesn't make sense?

Sorry to be so much trouble. I have never had a good understanding of antenna modeling. I truly appreciate yours and everyone's help.

 

[The DB]

Hey guys, if 2 - 1/4 wavelength ss whips or 2 - 1/4 wavelength wires were setup as a dipole and it shows in the image below...a match of <>72 ohms @ <>0 ohms of reactance at the feed point, then why then would a resonant Firestik a 1/4 wave coil of wire on a former...not show to be similar at <>72 ohms at resonance too?

The 72 ohms is with two 1/4 wavelength whips end to end. That we agree on. When you add loading coils, however, R changes at resonance.

DB are you saying you added a loading coil to shorten the legs to each side of a dipole and you saw 27 ohms of resistance at resonance in that setup?

Actually, yes, I did, it was near center, and I didn't make the entire loading coil, I used a raw inductance to simulate said coil. Because I was not simulating a real world antenna, just throwing something together for reference, that is where my disclaimer came from.

Are we to believe that because these two distinctively different 1/4 wave radiators over a good predictable (not a mobile) ground plane are not interchangeable?

Two different antennas, one a full length with no loading, and the other shortened with loading. Essentially two different antenna designs. Did you expect them to have the same results?

DB, you may be telling us this was a real world experience you had where you saw 27 ohms on a dipole, I'm not sure, but I would like to see this model if you did it using 4Nec2.

I am happy to post the data for you, like I said above, I used an inductance instead of a coil of wire for the loads, is EZNec able to do the same?

A simple resonant 1/4 wave helix for 11 meters takes too many wires for me to model in my version of Eznec, or I would build the idea and not ask it of you. Sorry!

I didn't make it that way, but I could, at least in theory. It is a project that I still intend to do at some point...


The DB

 

[Marconi]

Are you saying the wattage is not reduced or that my question doesn't make sense?

Pretty much!

71, you are referring to wattage here and IMO currents is the issue for modeling, there is a difference. If so, them I can only guess, deficiencies in antenna construction and installations probably does result in less currents at important points in the current distribution on antennas, just like you suggest.

My models do not model wattage (out) 71. I can control the wattage (in) however, but increasing the wattage (in) only increases the currents on the wires.

The performance results for gain and maximum radiated angle in modeling...does not show up as making any change. As an example in modeling, when increasing the wattage (out) like you would using your amplifier...there is no change in performance results, just the value of the currents on the wires change.

Wattage and currents are related, but these two antenna terms are not (the same) synonymous. However, DB can tell you the efficiency of and antenna...if he has it modeled using his 4Nec2.

My models did not show efficiency. This modeling stuff can be complicated and I could be wrong too, so don't get discouraged. I'm learning every day too.

 

[Riverman]

Pretty much!

My models did not show efficiency. This modeling stuff can be complicated and I could be wrong too, so don't get discouraged. I'm learning every day too.

Thank you.

 

[Marconi]

The 72 ohms is with two 1/4 wavelength whips end to end. That we agree on. When you add loading coils, however, R changes at resonance.

DB are you telling me your model with a coil is using the load feature in 4Nec2, or that you physically added the coil to the model? We recently discussed adding a physical matcher to a antenna models. I could be wrong, but I'm not too sure that adding a physical matching device to our models will produce good results. If so, then you may be right...adding a physical loading coil to an antenna model may not be a good idea either.

That said, the model attached here with a 10" loading coil added to a shortened radiator for a 27.205 MHz antenna with 4x102" slanted down radials at 36' feet above real average Earth, and it shows very little difference in the feed point impedance vs a full length 1/4 wave radiating element.

I will try and take the radiator with the coil above and replace the ground plane with a similar element below to make a HWCF dipole. If I do that you suggest the impedance at the feed point should show me about <>27 ohms of resistance at resonance, right?

Actually, yes, I did, it was near center, and I didn't make the entire loading coil, I used a raw inductance to simulate said coil. Because I was not simulating a real world antenna, just throwing something together for reference, that is where my disclaimer came from.

I'm not sure what raw inductance means. Are you telling me you used the loading coil feature in 4Nec2? If so, I get it...and maybe you are right after all. Below is a PDF file for what I get adding a physical loading coil to a shortened radiator with a good ground plane using 4x102" ss radials.

The first model is a shortened radiator with a 10" inch load near the top over 4x102" inch radials at 36' feet.

The second model is a the same ground plane with a full 99" whip in the top.

I added an overlay for these two for comparison. I was surprised to find the shortened radiator showing a bit better gain...I guess that is suggestive of the idea that top loaded antenna perform a bit better.

I will try adding a copy of this shortened radiator with a load as noted for the GP above and see if the match at the feed point shows me about <> 27 ohms and not 72 ohms like I argued above.

If I'm off track here and this is not an example of what you had in mind...then let me know, OK?

 

[Marconi]

Here is my model of the center fed dipole with shortened elements and a physical load near the tips.

DB, notice the match. It is not <> 72 ohms at resonance, but it is not <> 27 ohms either, not even close.

I suspect that you used the 4Nec2 load feature to make your load, but I'm surprised you saw <> 27 ohms. What am I missing here?

 

[The DB]

DB are you telling me your model with a coil is using the load feature in 4Nec2, or that you physically added the coil to the model? We recently discussed adding a physical matcher to a antenna models. I could be wrong, but I'm not too sure that adding a physical matching device to our models will produce good results. If so, then you may be right...adding a physical loading coil to an antenna model may not be a good idea either.

Neither. I have the ability to add a series load onto the antenna. This load can be inductance in Henries, capacitance in Farads, or resistance in Ohms, or some combination of the above. As I was short on time I didn't make two loading coils for said model, I would have if I had the chance. The two methods have shown similar results in the past, although notably not the same. The method I use tends to have results that are more optimistic when referring to gain for example, although that is not what I made said models for, I didn't even look at gain... Using an actual coil will show less gain, and have less of an effect on the R variable due to the losses that are present in using such a coil that were not factored into the method I used. And to make sure this is clear, for the third time, this result is specific to the model, the nature of the coil in question and where it is on the antenna will all have an effect on this, so I am not and have never said that that is the result you will absolutely get, it was an example from adding a coil in a specific situation.

I also want to point out that Firestik antennas have a different coil design that is unlike the loading method that I used, and said antennas may be designed to compensate for some of the effects of the inductance present in the antenna. If that is the case, both of our models are going to be way off.

That said, the model attached here with a 10" loading coil added to a shortened radiator for a 27.205 MHz antenna with 4x102" slanted down radials at 36' feet above real average Earth, and it shows very little difference in the feed point impedance vs a full length 1/4 wave radiating element.

Depending on the coil used that is possible. What we did were two different things with two different results. We didn't even start with a similar concept, or antenna length to use as a base. With so much differences between out models I don't find it surprising that we got two different results...

Some coils will have next to no effect, it depends on the coil and its implementation. All I know on your model is you used a 10 inch coil and have a ground plane made of angled radials. There are a lot of different possibilities that fits into it, but it doesn't look like you have much shortening offhand, whereas both sides of the horizontal dipole I used were five feet long. More shortening will mean more of a change in the R variable, and I'm pretty sure I used the five foot lengths because that length was mentioned for said antennas. In my model where I used two loads, one on each leg of a dipole setup, you can expect to have more of an effect and a model with one load used.

I did see you have a second model with two loaded sides as well, but again, I don't even know how long it is to begin with, much less other details.

Maybe if I get some time tomorrow I'll make a video showing what I am talking about, that might help you see what I did... Unfortunately that is a maybe, if I have time...


The DB

 

[Marconi]

DB, I have made and used such an antenna but always wondered and never knew what the impedence was as I don't own an analyzer.

71, a lot is written about the need for our antennas setups to show a perfect match. No one will argue that is important, but like I say in my quote in my signature line at the bottom of all of my posts,

We're just lucky that Mother Nature doesn't require our antenna systems to be perfect in order to work our radio."

71, there are some esoteric comments here on these forums that might try and prove you wrong in the results you describe...but don't be concerned...your ideas here are basically right in line, and IMO has worked for millions to get talking two-way. I get your point.

Maybe you miscued on a word or two regarding some modeling principal, but modeling has a fairly steep learning curve, so again don't be concerned. If you have a question...just ask and surely someone will post an answer. The answers may be good or they may not be so good however, that is for each to consider.

Again you're doing just fine and you are not that far off the track...like I am sometimes...when trying to describe what I see in words.

Thank you.

I accept your thanks, but if you have questions...then please ask.

Using modeling as an example, a full length center fed dipole mounted one wavelength above earth I get a feed point impedance of a hair above 72 ohms at resonance. Making both sides five feet long instead of tuning to a resonant length, and using an inductor on both legs (not quite the same as making an actual coil, but same principle) at resonance I now get 27.7 ohms. That can change based on the inductor's design, and where on the antenna it is placed.

I get it DB, and I too agree that the impedance at the feed point of a center fed dipole high above Earth should show about 72 ohms. I think maybe a none resonant 5' long radiator, like you describe should, with a proper helix coil, provide enough extra wire to provide this short radiator to show resonance and then provide a match somewhat close to what a full length 1/4 wave shows...assuming both have suitable ground plane of course and the radials are slanted down.

Inductors on the antenna lower R, but in a mobile environment, other factors raise R.

I can only guess but I think your model must have had horizontal radials to show anywhere near <>27 ohms of resistance at the feed point when resonant.

My thought was since mobile fiberglass antennas such as the Firestick are rated at 50 ohms, using two for the legs of a dipole would result in a feedpoint impedance of either 50 ohms, or 100 ohms (since there are two).

71 consider this. Two <>102" inch elements can be setup as a dipole at resonance, it will produce a <>72 ohm feed point resistance at the feed point just like you suggest. So, IMO if two smaller FS's are 1/4 wave radiators for CB...then setting them up as a dipole should also show us a <>72 ohm feed point impedance at the feed point or close. For this to happen and show a good match the GP radials must be slanted down and not horizontal. No amount of a helix wire coil will likely fix the mismatch for a substantially shortened 1/4 wave GP antenna.

Based on this I figure a typical mobile antenna could also turn out to see a ground plane that resembles slanted down radials...rather than the general accepted idea that mobiles reflect their GP as a flat surface...similar to horizontal radials. This is just my idea to try and explain, as best I can, what we might be seeing here.

DB, of course a helix wound coil requires its design, dimensions, and maybe placement, for it to work as intended...I don't argue any different. There might also be advantages in gain depending on the placement as well.

Since hams have been using hamstick dipoles for ages I figured this has been discussed many times and a foregone conclusion has been reached.

71, again you are right, and it does not take some esoterically worded explanation to understand that: A 1/4 wave radiator is a 1/4 wave radiator regardless of how it is designed and constructed.

 

[The DB]

I can only guess but I think your model must have had horizontal radials to show anywhere near <>27 ohms of resistance at the feed point when resonant.

Apples and oranges again...

Umm, no, and the descriptions I used to describe the antenna didn't support that either.

My model is a center fed horizontal dipole. There are no radials. Here take a look at this image of said model... The total antenna length is 10 feet, the circle in the middle is the feed point, the two blue boxes are the inductive loads, note I didn't make the coil directly, just added series inductance at those points like a loading coil would do.

Wait, why is it at 26.28 MHz? I didn't check something apparently, that would have affected the reading I got, but it is strange, the full length model that I copied this file from is at 27.18 MHz and I did not change it... Looks like I need to redo this model... After making the adjustment to frequency I still get an R of 28.9, so a change, but not a large change.

Here is the corrected one to show I fixed it...

And here the data for said model, the line of interest is the line called "Impedance"...

The DB

 

[Marconi]

DB, here is my 1/4 wave ground plane with a helix wound load on a shortened radiator that simply describes what you saw. This model is just a random design among many possible designs I could choose...that might show similar results to what your adding a "series load" into an antenna showing a resistance of <>27oms of resistance at the feed point.

Check out the Source Data Report below to see the model indicating an impedance near what you claimed. It would have help me understand your claim if you had simply let me know your model had horizontal radials.

Now, I'm no longer surprised at your results, and can support your findings and hopefully anybody looking at the models below can see what I'm trying to say here.

 

[Marconi]

DB, it would have help us understand if you had shown us your model in the beginning. I have not analyzed the dipole doing as you note here, but I will try that and get back.

I'm still curious, maybe you won't mind some more questions.

What is the difference between the two models above. They have different frequencies. What is significant about this difference and what did the first model ( the one with only one load on each side) show in the impedance on its main screen?

Are you telling me that the loads you added here are not using the load feature provided by 4Nec2, and that you created them using math?