Best Moblie Antenna for OTR Truck

[HARDROCK]

This document borrows heavily from several sources, at some points word for word, and there is no citation of said sources. It is like the guy is claiming this as his own work when most of it was taken from others, especially one M. Walter Maxwell, who's works were outright plagiarized in the writing of this document. And when it comes to said sources, especially the main source when it comes to SWR and complex impedance, I have quoted his works multiple times on this forum. You can get most of this from the original sources including a freely available .pdf file called "Another Look at Reflections".

Yes maxwell has excellent information.

27) With center-loaded mobile whips of equal size having no matching arrangement at the input terminals, best radiating efficiency is obtained on models having the lowest measured terminal resistance (highest resonant SWR, model for model). Models having lowest SWR are wasting power in the loading coil, because of either a low value of coil Q or excessive distributed coil capacitance, or both.

The above statement from maxwell applies to what i have found to work very well.

I checked my antenna yesterday at the feedpoint with the analyzer .

R 23 X 0 swr 2.1
R 49 x 0 swr 1.5 with matcher in line at the radio side of matcher.
R 50 x 0 swr 1.1 on the cobra 29 internal meter. which is not very accurate.
I placed a 100 ohm resistor on a pl259 and checked the 29 the meter showed 1.5
which if accurate should show 2.1 swr.



Tony
.

 

[fourstringburn]

Yes maxwell has excellent information.

27) With center-loaded mobile whips of equal size having no matching arrangement at the input terminals, best radiating efficiency is obtained on models having the lowest measured terminal resistance (highest resonant SWR, model for model). Models having lowest SWR are wasting power in the loading coil, because of either a low value of coil Q or excessive distributed coil capacitance, or both.

The above statement from maxwell applies to what i have found to work very well.

I checked my antenna yesterday at the feedpoint with the analyzer .

R 23 X 0 swr 2.1
R 49 x 0 swr 1.5 with matcher in line at the radio side of matcher.
R 50 x 0 swr 1.1 on the cobra 29 internal meter. which is not very accurate.
I placed a 100 ohm resistor on a pl259 and checked the 29 the meter showed 1.5
which if accurate should show 2.1 swr.



Tony
.

Try again and tune your antenna without a matcher and check with an analyzer.

If you can't get near the same results, you have a defective or poorly designed loaded CB antenna.

Also what you just posted goes against what you veen spewing all along about having feedpoint impedance being lower than 50 ohms and being ideal at 18 to 36 ohms.

So which is it now?

 

[HARDROCK]

Also what you just posted goes against what you veen spewing all along about having feedpoint impedance being lower than 50 ohms and being ideal at 18 to 36 ohms.

So which is it now?

No it does not. Maxwell states

With center-loaded mobile whips of equal size having no matching arrangement at

r
He states with No matching that has the highest swr at RESONANCE.
The key is resonance.
When i tune for lowest swr you are right the swr is 1.2 R 48 - 50 ohms.

But you also have lost effieciency because its not resonant for the frequency you are using.

Maximum power is radiated at resonance.

Do me a favor , Tune your antenna on the truck for resonance without any matching so that the R is 20 or so ohms and the swr is higher say 2.1 or 3.1.

Then let me know if your recieve is much better on the cb antenna.


Tony

 

[The DB]

I never once said or implied anything that disagrees with your new quoted statement, what I was bringing up was things like you using a source that talked about a shortened 40 meter antenna used in a mobile environment, that is an antenna that would have to be shortened by default, and using that information to say that a CB antenna, which you can use un-shortened in a mobile environment, had to have some sort of matching or you had a problem. For a 40 meter ham radio antenna, that would be correct, for a full length 1/4 wavelength CB antenna that statement is not correct. There is a very big difference between using these two bands effectively in a mobile environment, namely due to the size of said antennas resonant length.

One thing to consider, shortening a 1/4 wavelength CB antenna to, say, five feet, is a very different animal to shortening a 1/4 wavelength 40 meter ham radio antenna to the same length. In one case, the shortened version of the antenna is more than 50% of its original size, in the other case the antenna is less than 16% of its original size. One of these will have very minimal loading losses comparatively speaking, the other will have exponentially more loading losses. You can't use one antenna to definitively describe the losses of the other antenna. Apples and oranges. You, however, were trying to do just that.

Your half way there with that statement, however. To assume all of the losses (and their effects on R) in such a system in a mobile environment is in the coil isn't actually correct. Another thing to note, he (Maxwell) is comparing two antennas of the same length mounted at the same place, and running at the same frequency, the only difference in said antennas is in the coil designs. In that specific (hypothetical) case, the difference in R from one antenna to another will be 100% based on the coil. However, it is important to note that in the real world there are other things that also affect the R variable outside of the antenna itself. To assume R is not optimal only because of a coil in the "real world" is a serious error. You can take a magnet mount CB antenna and move it around on a vehicle and you will see R (at resonance) will change with where the antenna is on the vehicle, sometimes drastically. The coil itself is not changing, yet the tune of the antenna changes. The problem is you are lumping these other effects into the coil's effect, even though said effect is observable even without any coil on the antenna. In most cases, this will have even more of an effect than the coil itself. Explain this and what causes it and you will be near there. It is all in Maxwell's writings so keep reading.

Let me give you a hint with a formula.

R = R(radiation resistance) + R(coil loss) + R(something else)

Radiation resistance is, of course, the "good loss", or the loss of energy because the antenna is radiating a signal. When referring to an optimum R being 36 or lower, this is what said number actually refers to.

Coil loss is what you were talking about above, and for CB use drastically overestimating.

The something else is what you need to figure out. What affects an antenna in a mobile environment more than anything else. Actually any HF antenna in a mobile environment will suffer from these affects, the lower the frequency the worse the effect. By how you were writing things above, you were factoring this into the coil loss, which is a mistake to begin with as it exists weather or not a coil exists on an antenna.

---

Lets take a look at your numbers quick... If the R variable at resonance is less than 50, you can take 50 and divide it by your R reading to get SWR. If R is higher than 50 you take your reading and divide it by 50. These formulas only work at resonance, so if X is not 0, these are invalid. They are a good way to double check your readings.

R=23, X=0, SWR is 50 / 23 = 2.1739, your analyzer is near accurate there.

R=49, X=0, SWR is 50 / 49 = 1.0204, your analyzer has an SWR of 1.5? Problem. Either your R/X variable reading are not accurate, and/or your SWR reading is not accurate, and/or something else is affecting your analyzer.

R=50, X=0, SWR is 50 / 50 = 1, your reading from an claimed inaccurate device is within a reasonable margin of error. That isn't to say that all readings from said device will be reasonably accurate, but this one is.

100 ohm resistor, assumed X=0, at CB frequencies this X assumption should at least be close. 100 / 50 = 2. SWR on your analyzer shows 1.5:1 SWR? Problem. 1.5 SWR is generated by an R of 75 at resonance. Are you sure you have an X=0 when running this test? If you do then you have a problem somewhere.

I would be curious to what you are getting for the (magnitude of) Z and the phase angle (sometimes called theta, which is the θ symbol) for those measurements. You don't have to, its no big deal...


The DB

 

[HARDROCK]

Ok Thanks. That is the way it should be. I gotta go to work.
will get back when time permits.

Tony 73

 

[fourstringburn]

Mr. Hard rock, you are beyond ignorance to were it's not worth constantly repeating myself and I'll just ignore you instead.

 

[BBB]

Invest in one of these and Ye Shall See the Light... amen.

 

[HARDROCK]

That is good. I have the aa-30 rig expert.

 

[HARDROCK]

100 ohm resistor, assumed X=0, at CB frequencies this X assumption should at least be close. 100 / 50 = 2. SWR on your analyzer shows 1.5:1 SWR? Problem. 1.5 SWR is generated by an R of 75 at resonance. Are you sure you have an X=0 when running this test? If you do then you have a problem somewhere.

The above statement from maxwell applies to what i have found to work very well.

I checked my antenna yesterday at the feedpoint with the analyzer .

R 23 X 0 swr 2.1
R 49 x 0 swr 1.5 with matcher in line at the radio side of matcher.
R 50 x 0 swr 1.1 on the cobra 29 internal meter. which is not very accurate.
I placed a 100 ohm resistor on a pl259 and checked the 29 the meter showed 1.5
which if accurate should show 2.1 swr.



Tony

The 1.5 reading was the internal reading on the 29 swr meter. Not the analyzer.

 

[CTR]

Looking for or posting simple authoritative answers is not going to work. The entire issue is more complicated than people imagine. It would likely consume many days of writing for an almost meaningless result.

A claim has been made or implied that base resistance is a way to determine antenna efficiency. That isn't true, unless changes are confined inside clear boundaries. If, for example, we limit the physical size changes and only are allowed to change loading coil losses, prohibiting any other system changes, and if the antenna is length<1/4wl, a lower base resistance is indicative of lower loading coil loss. However...whether a lower base resistance might produce a noticeable or worthwhile change is another matter.

If we allow the rest of the antenna or antenna system to change, then all bets are off. Base resistance (or the real part of base impedance) tells us nothing.

The only sure way to determine an efficiency change is to measure the efficiency directly or in the most direct way possible. Base impedance is NOT the most direct method, but field strength and applied power certainly is.

As for analyzers like the MFJ, they are least accurate when reading low reactance values close to 50 ohms. This is because they calculate reactance by measuring and comparing the scalar values of four voltages, generator source voltage, 50 ohm bridge balance (Vr), voltage across load (Vz), and voltage drop across a 50 ohm series resistor to the load (Vs). When Vs and Vz sum to exactly equal source voltage, the load is non-reactive. The issue is when close to 50 ohms, just a one or two bit error in Vs or Vz can result in a few ohms showing as reactance when there isn't any reactance. The least accurate measurement is reactance near a 1:1 SWR.

This is, however, unimportant. Small values of reactance are meaningless, as is being slightly off resonance. The notion system efficiency peaks at antenna resonance is also false. Making things worse, all feedlines are not exactly 50 ohms and many are a good bit off. I've seen new reasonable quality cables as much as 2-4 ohms off, and then we have how people make their connections and common mode issues. Then when we consider how very small any efficiency changes are once we have a remotely reasonable SWR, a lot of time is spent arguing what amounts to nothing.

I'm not even sure what people are debating here, and why they are debating it. If it is tuning an antenna mounted in a given spot, just adjust it for lowest SWR. If the SWR is too high for the radio when that is done, do some sort of matching system. With a given antenna that is even remotely close to 50 ohms real part at the feedpoint, there will be immeasurable efficiency changes whether tuning for resonance or SWR.

If the antenna base impedance is too low, a shunt reactance of L or C and tuning the antenna for 50 ohms or a matching transformer produce provide virtually identical results, provided the added components are not lossy.

Arguing about how many angels fit on a pin head just wastes time. No one will notice a fraction of a dB change.

 

[HARDROCK]

The below is very Good advice . Thanks to Tom w8ji consultant engineer.

That just isn't true as a blanket statement.

Resonance does not determine efficiency.

Efficiency is dictated by (radiation resistance/radiation resistance + loss resistance)times 100 when both are normalized to the same measurement point.

Nowhere in that is resonance.

There are cases where efficiency peaks near or at resonance, and there are cases where it peaks off resonance. There are cases where lowest SWR might be best, and cases where resonance is best, and cases where efficiency peaks between the two.

You can believe whatever you like, but categorically stating "resonance is highest efficiency" is wrong. Lowest SWR is not guaranteed highest efficiency when adjusting a mobile whip system, but it is the easiest and most accurate thing to tune for and with rare exception is better than tuning just for resonance.



----- Original Message ----- From: "Tony" <tonywhite105@gmail.com>
To: "Tom W8JI" <w8ji@w8ji.com>
Sent: Thursday, November 10, 2016 12:43 PM
Subject: Link to question


http://www.qsl.net/g3tso/Hombrew-Mobile Antennas.html


Tom this is why i wanted to talk to you. I used the above method for
my mobile radio. He is DEAD ON!!!!


What i dont understand is everybody states tune for lowest swr and
forget resonance.


Why is it when i tuned with a matcher placed at the antenna feedpoint
That my recieve and transmit is alot better ?


Tony



---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus

 

[HARDROCK]

Thanks again for w8ji advice. The below is from Tom

Many high gain high efficiency antennas or antenna systems are technically not "resonant antennas". Most AM broadcast towers are not resonant lengths, although most are tuned and matched at the base, there would be little difference if they were matched some distance away from the base.

Most of this drama is in probably in people's heads, and not worth the effort. It can all be handled mathmatically and it will work out perfectly with real life.

The reasons I tell you to tune for lowest SWR is:

1.) Reactance is the least accurate measurement in an analyzer

2.) SWR is the most accurate measurement

3.) The radio generally wants lowest SWR when transmitting

4.) The radio sets the feedlinme SWR, NOT the antenna, when receiving. There is almost no change in S/N with antenna SWR over modest change values, say under 2:1 or 3:1 on HF.


An antenna radiates from current. The voltage is only necessary to cause the current to flow. The phase and amplitude of current determines pattern. It is current distribution, or ampere-feet, that matter.

Think about this....

Why does an extended double zepp antenna have more gain than a full wave when the EDZ is not resonant, and is usually matched with a tuner or stub far from the antenna??

Why is a Yagi, with elements that are not resonant and do not have voltage and current in phase, a good efficient radiator?

You have people with no concept of how an antenna actually works making up rules.


----- Original Message ----- From: "Tony White" <tonywhite105@gmail.com>
To: "Tom W8JI" <w8ji@w8ji.com>
Sent: Friday, November 11, 2016 12:22 PM
Subject: Re: Link to question


Is it true at resonate frequency the antenna generates the maximum voltage
across the antenna and the maximum current through it with the correct
phase relationship ?

Tony

On Nov 11, 2016 8:24 AM, "Tom W8JI" <w8ji@w8ji.com> wrote:

All of the loss change from not being exactly at resonance is either in
the feedline or the additional matching components. It is immeasurable
negative change unless the base SWR get really high and the compensation
circuit or feedline is pretty lossy.

Many high gain high efficiency antennas or antenna systems are technically
not "resonant antennas". Most AM broadcast towers are not resonant lengths,
although most are tuned and matched at the base, there would be little
difference if they were matched some distance away from the base.

Most of this drama is in probably in people's heads, and not worth the
effort. It can all be handled mathmatically and it will work out perfectly
with real life.

The reasons I tell you to tune for lowest SWR is:

1.) Reactance is the least accurate measurement in an analyzer

2.) SWR is the most accurate measurement

3.) The radio generally wants lowest SWR when transmitting

4.) The radio sets the feedlinme SWR, NOT the antenna, when receiving.
There is almost no change in S/N with antenna SWR over modest change
values, say under 2:1 or 3:1 on HF.


Tony 73

 

[HARDROCK]

Ctr agree that fs and applied power is.
I would like to hear your comments on tuning the mobile antenna via FS METER

Tony

 

[The DB]

CTR, not a bad post for a first post on the forum. And it is mostly true.

A claim has been made or implied that base resistance is a way to determine antenna efficiency.

It isn't an absolute method, but it is an indicator, and it gives a reasonable idea. If you get good at it, you can also predict to a point what various changes will have on R, and if something is way off you know you likely missed something. To say it is completely pointless, with all due respect, is a bit much.

That being said, I agree that there is more in play when it comes to R and the various parts of an antenna system that affect it. To many people, R is simply R. It doesn't mean any more to them than that. Getting people to realize that there is more here is why I used that highly, and really over-simplistic explanation. If someone wants to learn various details about the how and why, they can start with that explanation, and we can build from there. Sure, it might affect an antenna insignificantly most of the time, that doesn't mean that some of us don't find it interesting...

This is, however, unimportant. Small values of reactance are meaningless, as is being slightly off resonance.

This statement is true, if not a bit understated. It is true that resonance, like SWR, has little to do with maximum radiation from an antenna. It is, however, beyond the limits of a lot of people's knowledge. Resonance being used as this point is the logical progression that many people follow. It is a big step for many to go beyond this point, so I generally try and stay within it when giving advice.

When you are talking about tuning for SWR and being done with it, that is where you are coming up short. You can have a low SWR and still have a crappy antenna. I've seen an antenna (with a problem) have a low SWR over 100 MHz of frequency, it meets you low SWR requirements, but the antenna was really more like a dummy load. Yours is a method that will work much of the time, which is why the average Joe CB Guy can get away with using it, however, if you have a problem SWR may not be enough to help you resolve it. SWR only tells you so much, and that includes almost nothing about the antenna itself. It is really only good for two things, to ensure that the coax is safe to plug into your radio, and to use as an indicator of something changing in the antenna system. If something does change, or you found that magic low SWR spot when tuning and the antenna still isn't working well, it won't help you any. SWR is, in and of itself, a combination of good bad and irrelevant parts. If tuning for SWR was really all that mattered, we would all buy dummy loads and be done with it...

The reality is, you need to understand all of the variables in play, and understand what they mean, to ensure an antenna is working as it is able to. Or, you can limit your self to prayer and hope...

Arguing about how many angels fit on a pin head just wastes time. No one will notice a fraction of a dB change.

In the cases described above, this is mostly true, however in the use-able SWR range in a mobile environment the amount of change in an antenna can be several dB worth of gain. I've seen an antenna that was experimented with using a field strength meter, and in that 1.5 SWR range went from maximum full scale to off the low end of the scale two times. Now it wasn't (unfortunately) one with a dBm measurement, just a cheap one with an analog scale and an analog dial, and just tuning the antenna around in that SWR range had that much of an effect on the field strength of the antenna. I'm sorry, but with that experience, I will not limit myself to one inadequate reading when tuning antennas, mine or otherwise.


The DB

 

[Mudduckmobile]

I hope I have this set up right now... I'm on lsb.385, and usb.555 the most as a rule not lower or higher. This was today at 4400ft above sea level, Humidity was 73%. I was on talc like volcanic ash soil and not even a tree with in 1/2 mile, and no hills or buildings within 5 miles. (photos of antenna where taken in town).Readings done with mfj259c 16in from antenna feed point.