A question on whips...

[Captain Kilowatt]

984 versus 1005........the difference is 0.8 inches in a whole wavelength. Since tuning will be required regardless of what numbers you use I'll go with the one that leaves a TINY bit more to trim just in case however since we do not live in free space it is moot.

 

[Wire Weasel]

Short Answer Is: All installations/setups are different. Put the 102 on your vehicle where and how you are mounting it and see how it does. If great SWR ..... leave it alone. If you need to lengthen it to get the SWR right in your desired freq. coverage .... then switch to a 108 or add a spring or shaft extender.

 

[Handy Andy]

There has to be something I am not getting.

I'm only going to show this...because with everything else being equal...

Meaning RESONANCE - all elements used are EQUAL length...

Ok, one thread did not seem to help me get my point across.

It deals with how FEEDPOINT Impedance versus SWR - even if RESONATE - why SWR?

Look back at this post...

https://www.worldwidedx.com/threads...-best-antenna-performance.262791/#post-775830

The Graphic in there is what I want you to think about - for if ALL THINGS except orientation of elements are equal, then FEEDPOINT Impedance and the SWR from it - does it become moot? As in discardable?

Read the rest of the thread for there is a device on page 4(?) that does a wonderful job of fixing specific things wrong with SWR even when everything else is equal.

We keep trying to put the cart before the horse...

Because with a 102" whip being the physical length and its construction - is not exactly favorable for any type of easy or simple install - it has needs - so we must put it in an ideal spot - not the spot we want it to be - which makes the answer your question "What Am I Missing Here?" Not a simple as it seems - for there are many factors we must deal with but location being one of them, cannot always make it work well for the situation were in.

Ok, we can't put a 102" whip on top of a semi - well we could - but the DOT is going to have a say or two about that once we start seeing the number of damages to street signs, streetlights and even stop lights - that whip will thwack against - to the point of both members the antenna and the object it strikes - will be compromised and not work well anymore - at least, effectively.

​

So, I just wanted to help answer your question by giving you a carrot - and then you can run down any rabbit hole you choose to feed your desire...

 

[Handy Andy]

Ok, with the above in mind...

Did you ever stop to think about what "Equal" in all of this means?

Well, in your understanding (if you can allow) the "Marconi antenna" approach as having only 1 driven element - and let the mirror come from the counterpoise or earth itself - then you also see where specific problems occur with that application.

• Not all vehicles are created equal.
• Marconis antenna design is considered to be a Monopole.

So, one element can be made resonate - but the other half? Well, you just can NOT trim a car down to equal length...(Chain saw or otherwise)

Once you understand that approach - the location for lowest SWR present at the time - may indicate the best location or best effective radiated power from that location? Your choice...

• reciprocal comes to mind.

The vehicle has a lot of metal - so it (Being the opposite element) will appear as a very LARGE capacitor interactive with the Resonate element (one plate or element is not equal) - so the Picture on Page 4 - shows what @nfsus uses to solve that condition. To cancel out an otherwise very large capacitance problem making the feed point impedance you would otherwise have - see 50 ohms for the coax to connect to. properly so it can transfer it's energy from the radio to the antenna and reciprocate back to the radio FROM the antenna

Welcome to Radio.

 

[Greg T]

Aaahhhh yes!! The age old discussion about 102" vs 108". I'm no expert and certainly no tech, BUT, I have decades of whip experience! ( sounds kinky)

I've installed at least 100 whips in my days, and have NEVER had 102" load up like 108". This is the one time where all that math fails, terribly. I've mounted whips on bumpers, side panels, special brackets, tool boxes, and many more. Yes, had one on my own vehicle in the middle of the roof. In EVERY case 108" loaded much better than 102". AND, I ALWAYS use the shortest length of coax I need to get the job done. None of this crap about 3', 18' or any of the other wives tales.
I've asked numerous good, established technicians to explain this to me, but, no joy. My very good friend was a chief engineer and RF specialist working for the local radio and TV stations, doing maintenance, repairs, and transmitter installs. He knew his stuff. Can't explain why 108" is the magic number. The math doesn't work.

 

[secret squirrel]

There is a lot of wise tales out there passed around. Like others I start with the disclaimer that I am not a tech. However I a have witnessed what works. I truly believe that to work properly the 102 inch whip needs the 6 inch spring and the ball mount. My experience with the installs that with all three parts I have had luck getting a useable SWR with a nice curve across the CB band even before properly trimming the antenna. Another wise tale I witnessed is an experienced tech explained to me one day was it you are forced to install in a bad mounting location, then use 21 feet of coax. The example he was showing me was that if you mount on the front bumper corner of a Triaxle, then go with 21 feet of coax. My best guessament is that it just tricks the meter. If any can elaborate about the 21 feet and poor ground plain please do so.

 

[freecell]

984 versus 1005........the difference is 0.8 inches in a whole wavelength. Since tuning will be required regardless of what numbers you use I'll go with the one that leaves a TINY bit more to trim just in case however since we do not live in free space it is moot.

1005 ÷ 27.185 = 36.968
984 ÷ 27.185 = 36.196

36.968 - 36.196 = 0.772 feet
0,772 X 12 = 9.264 inches.

the difference is 9.25 inches, not 0.8 inches.

 

[guitar_199]

Thank you gentlemen for the comments and for the reference to what appears to be a great thread and I WILL study it. I saved the link in my browser so that I can find it a little later.

That said, I am about to go in and watch my team find "new and exciting" ways to lose a football game.

 

[freecell]

Short Answer Is: All installations/setups are different. Put the 102 on your vehicle where and how you are mounting it and see how it does. If great SWR ..... leave it alone. If you need to lengthen it to get the SWR right in your desired freq. coverage .... then switch to a 108 or add a spring or shaft extender.

the swr on the feedline has nothing to do with the fact that @ 102" your antenna is operating in the 10 meter amateur band. you've allowed your education by antenna manufacturers to interfere with your learning.

feedline swr indicates whether or not the antenna is matched to the feedline. it never has been, isn't now and never will be an indication that the antenna is operating at any particular frequency.

246 ÷ (102") 8.5' = 28.941 mhz., 246 ÷ (108.588") 9.049' = 27.185 mhz..

 

[nfsus]

Yes the car is the other half. The counterpart to the vertical. You're 108 is tuning different due to the ground plane of both the vehicle and the ground around the vehicle. Take any antenna and raise it or lower it and it will change how the element radiates. That's my thought

 

[freecell]

Everywhere I go... I see people saying that when you use a 102" whip... you need to add a 6" spring to get the correct length of 108".

There has to be something I am not getting.

If one considers that a halfwave is 468/f in mhz.... then for the middle of the band... it would be 468/27.185 which is...

17.22 ft ... or ...... 206.58"

Divide that by 2 for a quarter wave and you get...

103.29".

If you turn the math around....

A 108" whip .... twice....is.

216"..... or..... 18 ft exactly.

468/18 would be 26 mhz.

Why is it a popular opinion that the total length of the whip should be 108"?

What am I missing?

Thanks!
Bob

492 ÷ 18 = 27.333 mhz..
492 ÷ 18.098 = 27.185 mhz.

1.176 inches for a shift in frequency
of 148 khz. or almost 15 channels.

 

[Captain Kilowatt]

1005 ÷ 27.185 = 36.968
984 ÷ 27.185 = 36.196

36.968 - 36.196 = 0.772 feet
0,772 X 12 = 9.264 inches.

the difference is 9.25 inches, not 0.8 inches.

Yet another reason I shouldn't be sneaking my phone out while on the plant floor in contravention of company policy. Too easy to make mistakes while trying to hide the fact that you are using it where you shouldn't be.

 

[nfsus]

Want to test it out? Loosen your ball mount and start to lean the antenna slightly forward and backward while watching your analizer. Just a little. Half inch or so. I'm my case with the mount on the side of the tool box as I lean it forward it puts the spring closer to the tool box and also puts the antenna more over the center of the truck. As the antenna goes forward or backward it changes every parameter. Sitting parked I can loosen the mount, lean it forward, tighten it down and use the coupling effect and the position over the truck to perfect my readings. Works perfect. Till you drive of course then as it moves out of position it changes.

So 108 or 102? Depends on mounting.

 

[freecell]

the resonant frequency of an antenna has nothing to do with "all elements being of equal (physical) length."

a radio antenna is a form of tuned (L/C) circuit consisting of inductance and capacitance and as a result it has a resonant frequency. this is the frequency where the capacitive and inductive reactances cancel each other out. at this point the antenna appears purely resistive, the resistance being a combination of the loss resistance and the radiation resistance.

https://www.electronics-notes.com/a...gation/antenna-theory/resonance-bandwidth.php

once the resonant frequency of any antenna has been determined, increasing the operating frequency causes the antenna to become inductively reactive (too long) and decreasing the operating the frequency causes the antenna to become capacitively reactive, i.e., too short.

every type of an antenna is an L/C circuit.
the output tank of a transmitter is an L/C circuit.
any feedline, coaxial or parallel line is an L/C circuit.

L = inductance (current)
C = capacitance (voltage)

 

[Handy Andy]

a radio antenna is a form of tuned (L/C) circuit consisting of inductance and capacitance and as a result it has a resonant frequency. this is the frequency where the capacitive and inductive reactances cancel each other out. at this point the antenna appears purely resistive, the resistance being a combination of the loss resistance and the radiation resistance.

One thing you have not satisfied in ANY of these arguments is the ASPECT of Elements to each other - physical and the length was used to simplify the thought process for you and anyone else.