Re: No ground plane antenna needed for fiberglass

magnetics and Maxwell's stuff is the class im finishing right now, except its Physics and not an EE course.

good stuff though

as long as we agree on the 'no change' by changing the length of coax then im happy....... since we have TOTALLY hijacked this post.... /ubbthreads/images/graemlins/smile.gif /ubbthreads/images/graemlins/blush.gif

 

Re: No ground plane antenna needed for fiberglass

If your antenna is not properly matched, standing waves will be set up along the transmission line. You can fool yourself in thinking you've matched the antenna by shortening or lengthening the coax, and in doing so, simply finding a position along the coax at which the standing wave is non-existant. Sure, you match the antenna and transmission line to the transmitter, but you also greatly reduce your efficiency. As mentioned previously, a properly tuned antenna will not produce a change in apparent SWR as you change coax length.

 

Re: No ground plane antenna needed for fiberglass

[ QUOTE ]
...If your antenna is not properly matched, standing waves will be set up along the transmission line...

[/ QUOTE ]

Tim, that's somewhat true. The power will be sent via the cable to the antenna. The antenna in turn will REFLECT back the power, due to the mismatch, and that will be seen in the cable as standing waves. If the impedance match is bad enough... the reflected power can be induced back into the final power amps and burn out the amps (unless it's protected).

[ QUOTE ]
And to DDWAG16 --

It may take an RF/propagation engineer to settle this. Those classes were about on par with magnetics and Maxwell's equations...(which makes RF look like simple addition)

[/ QUOTE ]

I was a Propagation Engineer back in the '60s!

 

Re: No ground plane antenna needed for fiberglass

So what kind of antenna do you need on a glass bodied Jeep? /ubbthreads/images/graemlins/headspin.gif

 

Re: No ground plane antenna needed for fiberglass

[ QUOTE ]
So what kind of antenna do you need on a glass bodied Jeep? /ubbthreads/images/graemlins/headspin.gif

[/ QUOTE ]

IMHO... a 1/4 wave length antenna usually will do quite nicely if you're after Jeep to Jeep communications.

If you want to be able to get out further, then a 5/8 wave length antenna would be the best bet.

As for a manufacture... I like Firestik. They've been around since dirt and are well respected in the marketplace.

 

Re: No ground plane antenna needed for fiberglass

Interesting -- all of it.

Sooo
- with a 102" steel whip antenna and 6" spring - which is 1/4 wave for CB's - no loading coil whatsoever.
- And with a 1/2 wave length of coax to the back of my Jeep,
- the coax shield grounded at the back and at the radio
- and an SWR meter attached at the transmitter
- and a RF power meter attached to the xmtr side of it

Cutting and trimming the antenna end of the coax an inch at a time DID NOT GET THE SWR LOWER BY A BUNCH AND PRODUCE AN INCREASE IN POWER?

The meters must have mind of their own!

SWR got much lower, and power nearly doubled according to the meters.

No perceptable difference was noticed in reception, but a guy down the block thought I turned on my linear.

So much for theorey vs. reality.

I got a neighbor that swears his Pontiac will go where Jeeps "are afraid to go" too.

 

Re: No ground plane antenna needed for fiberglass

So, with all that's been said...I still don't know how to lower my SWR...

On my Jeeps I read about 1.3-1.5 on chanels 1, 20, and 40. But on my tow rig I get about 2.1 on all three chanels...How do I go about lowering the SWR?

 

Re: No ground plane antenna needed for fiberglass

This link Cobra SWR is to the Cobra CB site....I would think they know a little bit about CB's....

 

Re: No ground plane antenna needed for fiberglass

And here's some Firestik info

http://firestik.com/Tech_Docs.htm

 

Re: No ground plane antenna needed for fiberglass

I looked at the attachment and it's pretty nasty on the left side. The black blotched out some of the print. I can't read all of it but as I suspected, the section you quoted out of is receive mode, with the receiver as the load. I suspected that from the sentence, "If the load is purely resistive and equal in value to the characteristic impedance of the line, there will be no standing wave." The totally resistive load I can think of are receiver, dummy load or we're talking theory only.

So, from your quote, "In case the load is not purely resistive, and/or not equal to the line Zo, there will be standing waves. No adjustement that can be made at the input of the line can change the SWR, nor is it affected by changing the length of the line." It is definitely referring to the wrong cable Zo characteristic line impedance connected to a receiver as a load. But then as stated, when dealing with a totally resistive load, there is no SWR problem so unwrap a coat hanger and stick it in the hole. You'll receive, maybe not well, but you'll receive. DO NOT hit the transmit button.

In transmit mode, the antenna is the load and the section AFTER the portion you quote from is titled, "The Antenna as a Load". Now here is where we have to match the antenna as a load to the transmitter.

So,

[ QUOTE ]
if "that statement" refers to what i took from the book, then you mistook it.

there no such thing as improper line impedance, only improper antena matching (unless you have a damaged line, but we ignore this)

[/ QUOTE ]

You are the one who made the mistake apparently not realizing that the section you were quoting from was with the receiver as a load and trying to misapply comments made there to the antenna as the load.

[ QUOTE ]
hell, i could feed a 50 Ohm antenna with 75 ohm TV cable with no problem if i can match the antenna (by means of a balun, transmatch, or matching network)

[/ QUOTE ]

You can do no such thing. You say "feed a 50 Ohm antenna" so you're talking transmit mode and the subject of this thread is CBs. So to match it to the transmitter, you still have to have the 50 Ohm impedance at both ends. You could try matching it to anything in the middle. This was done for years with television matching a 300 Ohm impedance antenna to a 75 Ohm with a matching transformer and then another transformer to match the 75 Ohm back to 300 Ohm. Then a splitter was used to split out the VHF and UHF.

[ QUOTE ]
soooo what yur saying, in short, is that we're right, taz is wrong

[/ QUOTE ]

Where did you get THAT out of THIS???

[ QUOTE ]
EE at the Univ of Texas
Computer Science at Cal State

Impeadance is very important....the transmitter will have characteristic impeadance...typically 50 ohms. If the load (coax and antenna) does not match, then not all of the power is transmitted to the antenna....in this case, the comparison of low back pressure does not apply. The only thing a longer coax does is increase loss slightly.

Basically, if the load impeadance (coax and antenna) matches the TX, then there is minimum power loss at the CB and it is able to send it's maximum power down the line. If the impeadance is higher or lower, then part of the energy is 'left at the tx'.....This is where we get into VSWR (Voltage Standing Wave Ratio)...or in laymens terms, the ratio of transmitted power vs reflected power. If the load (impeadance) does not match the TX (source), you get reflected power.

Based on the median freq of CB's, even a 1/4 wavelength antanna would be too tall...hence, we have 'loaded' antenna...which usually come in the following flavors: (the loading is usally a coil of wire to make the CB 'think' that the antenna is the right length)
Bottom Loaded - the coil is at the bottom
Middle Loaded - the coil is in the middle
Top Loaded - the coil is at the top
Fully Loaded - no coil, the wire is the length of a 1/4 or 1/2 wavelenght but is evenly wrapped around the antenna. Typically this is your best choice.

It is very important to understand that RF antanna propapation is a science all it's own....there is a reason they call it FM (F*&$ing Magic). By comparison, microwave is easy...feed lines are rectangular tubes 1/4 x 1/2 wavelengths in size that dump into a feedhorn that is aimed at a dish.

In the old days of mobile CB's, they usually consisted of a base mount (with a coil of wire in it) and the antenna which was just a stiff wire that plugged into the base. To adjust the VSWR, you would remove the wire and cut a small piece off to shorten it...this was adjusting the effective impeadance. Cut it too much and you had to go buy another one.

The critical point in the system is where the coax meets the antenna...this is where we transition from signal transmission down the coax to propagation out the antenna. If we assume the coax has the proper impeadance, then if the antenna is the proper length and impeadance...the maximum available RF energy is transmitted.

Or we can look at real examples....what about all the hand radios that police and fire use? We have radios that have the antenna on the main radio, and others with the antenna on the mic. And if you look in the trunk of the radio cars, you don't see a bunch of coax coiled up to keep the length right....

--------------------
John
84-CJ7, 89 Cherokee

We learn by our mistakes...Thats why we are all so smart.

[/ QUOTE ]

You seem to have a habit of reading things that aren't there and/or misapplying what is there.

[ QUOTE ]
i'll be waiting on the "oh, i stand corrected" post from taz....i think i'll be waiting a long time...i wont hold my breath

[/ QUOTE ]

That happened one time and it was a typo where I added an extra "o" to "loser' making it 'looser". I really didn't think you'd hold that against me forever. As long as we're on the subject, are both of you Shift keys broken? Do you not have a spell checker? I compose in MS Word and whenever I paste something from one of your posts in, my spell checker goes nuts, underlining a bunch of it in red.

[ QUOTE ]
One way to find out is take two jeeps....both with the same CB and same length of coax. Drive apart until you start to loose signal. Now, change the length of coax on one jeep and retest reception. My bet is 'no change', assuming all other factors stay the same.

[/ QUOTE ]

No not really, we need to get back to the original statement that I disagreed with:

[ QUOTE ]
And contrary to popular mis-belief, the coax length is not an issue....I've heard some people say that you can't shorten the coax because that will upset the VSWR.....WRONG. The coax has a fixed impeadance....regardless of the length...which is differnt from signal loss.

[/ QUOTE ]

That means set up a CB with an antenna (I'd prefer 1/4 wave whip), get the VSWR set and then start cutting off cable and see if the SWR meter shows a change. Maybe cut in 6" to 1' sections until you have cut off 18' or approximately a half wavelength checking after each cut.

[ QUOTE ]
Interesting -- all of it.

Sooo
- with a 102" steel whip antenna and 6" spring - which is 1/4 wave for CB's - no loading coil whatsoever.
- And with a 1/2 wave length of coax to the back of my Jeep,
- the coax shield grounded at the back and at the radio
- and an SWR meter attached at the transmitter
- and a RF power meter attached to the xmtr side of it

Cutting and trimming the antenna end of the coax an inch at a time DID NOT GET THE SWR LOWER BY A BUNCH AND PRODUCE AN INCREASE IN POWER?

The meters must have mind of their own!

SWR got much lower, and power nearly doubled according to the meters.

No perceptable difference was noticed in reception, but a guy down the block thought I turned on my linear.

[/ QUOTE ]

My experience was similar with the same whip with the same spring on the back of a Lincoln Mark IV.

SWR went down from over 2:1 to nearly 1:1 and the transmit meter on the radio showed higher output.

[ QUOTE ]
So much for theorey vs. reality.

[/ QUOTE ]

The theory I learned matched the reality. The installation book with the CB, my EE class, magazines and most CB'ers all predicted that same result.

[ QUOTE ]
This link Cobra SWR is to the Cobra CB site....I would think they know a little bit about CB's....

[/ QUOTE ]

Cant argue with that, but near the bottom: /ubbthreads/images/graemlins/deal.gif

Under section: I can't get my SWR down

Subheading: If you can't get your SWR below 3:

B. The antenna cable has been shortened or lengthened. Changing the length of the antenna cable can affect the SWR. If your antenna came with cable, DO NOT shorten the cable. If your antenna did not come with cable, try using an 18-foot cable. If you have extra cable, do NOT tightly coil the extra length. You can run it back-and-forth inside the vehicle, or coil it in at least a 12 inch diameter loop and then tape the middle together into a bow shape. If you need to lengthen the cable, use an in-line coupler and add lengths in multiples of 3 feet. You can also contact the antenna manufacturer for recommended lengths.

Sounds to me like they know what they're talking about. And thanks, Dawg. /ubbthreads/images/graemlins/40BEER.gif

So now I'm right and Greg is wrong?

Are we done now???? /ubbthreads/images/graemlins/sleep2.gif

 

Re: No ground plane antenna needed for fiberglass

I think we should continue this discussion over a few cases of good beer.....man, I wish I could make it to your bash.....oh well....

 

Re: No ground plane antenna needed for fiberglass

One must understand the princple of nodes and peaks.
A node is where the voltage never rises, the peak is where it oscillates to the maximum value.

In a tuned circuit, or exactly an even multiple of wavelength, those peaks and nodes occur at the same places along the wire all the time.

A full wavelength of wire (either antenna or coax to the antenna) has a node (minimum voltage) at each end and one in the middle at the half way point.

It's easiest for the transmitter to feed into a 0 voltage - node, than something of a higher voltage (reflected energy or back pressure.) So the preferred method is to feed into a node. That's concerning resonance.

Impedance is the AC resistance in the cable or antenna - different than DC. 50, 75, 300 ohms etc cable impedance is essentially not related to resonance. It's what the transmitter will "see" when it "looks into" the cable.
The transmitter output needs to be matched to the cable impedance. But that really has nothing to do with resonance (OK, very slightly.)
And, for maximum transfer of energy, the antennas impedance needs to "be there" too.

Doesn't it make sense to have the transmitter "pushing" it's power into someplace that not "pushing back?" (reflected power.) That's why resonance and nodes come in.

As above, a full wavelength wire or combination coax and antenna has 3 nodes. Ends, and middle. Total both ways is 2 wavelengths, forward and reflected, xmtr feeds a node.

Now cut that exactly in half - still one node on each end. The transmitter is still always feeding a node - good!

But if you cut that in half again, so it's 1/4 wave total, you end up with the xmtr end feeding a node one moment, then feeding a peak on the next 1/2 cycle. Feeding into the reflected peak will transfer very little, if any, power. So you've effectively cut the power in half or less.

So -- the "total" has to be an even multiple of a full wave. It has to oscillate or resonate so the transmitter is feeding the lowest reverse voltage (back pressure.)

Now for the transition portion - coax to antenna.

IF - IF - the coax is grounded at the antenna end, it must follow the above - the coax has to be an even multiple of the full wave. Reason is it's just like at the transmitter end - it has to feed into a node - from a node to get max energy transfer. Then the antenna has the maximun power in it, and can radiate effectively -- IF it can resonate with a node at each end too.
Often the coax is 1/4 wave, the antenna is 1/4 wave, then when the energy reflects back it travel those same 1/4 waves - resulting in the 0 node at the transmitter (0 backpressure).

BUT -- if the coax is NOT GROUNDED at the antenna end, then the coax + antenna length has to follow the rules, the TOTAL, rather than individual components.

Sooo - you could say in some cases coax length is not critical, in others it is - depending on HOW IT'S GROUNDED.

The magnetic based antennas have no ground at the antenna end. Those the TOTAL has to be right to get maximum transfer.

Firesticks and the like have a ground wire that should be attached at the antenna base. That makes the coax need to be 1/2 or full wave or a multiple thereof to feed it correctly. Sure, a random length coax will work, but if you want maximum power transfer, you have to follow the rules of physics.

Some Firestick and the like have a slider or adjustment that "tunes" the system a little. It's usually just a slug or shorted turn. It's not really "matching" the resonance to get the nodes in the right place, but it is enough resistive load (the slug or shorted turn absorbs some of the energy) to trick the entire system into thinking it is. Slightly off can be compensated by that adjustment. But look at the SWR when it has to be "adjusted" very far to dial it in. You can get it down, but not even near the "ideal" 1:1.
Do it right and you get much more.

Recieving is a different thing altogether. Ideally it would be at a full wavelength - or multiple, but since there is so very little power, impedance matching and resonance don't play much of a part. Reception voltage values are measured in 10th of Micro volts - that's 1/.000001 volt. A long random length wire works for most practical applications.

But the debate -- you all are correct - just dependng on what type antenna/coax you are driving!!!!!!

Try something -- get one tuned to the best SWR. Then simply disconnect the ground at the base. Notice the SWR change. Now the TOTAL is different because the exact total length is wrong.

But -- using it ungrounded at the far end - the coax is trying to become part of the radiating system, but the signal can't get out due to the grounded shield. Max power can be fed into it from the xmtr, but it still doesn't radiate very well. That's why the magnetic based types don't get out very well.

Put on one of the "loaded" shortie magnetic bases designed for a plastic "no ground plane" motorhome - you are lucky if you can transmit across the street. Yes, you can still recieve, but you can't be heard.

TRY it!!!!

 

Re: No ground plane antenna needed for fiberglass

Taz, if you want to discuss this more PM me and i'll shoot you my phone number and we can go from there.

i've had a lot of hands on with this, and can rest assured that i didnt mis read the book, nor is it refering to recieving only or anytihng of the sort,

im at my moms for mother day so i dont have proper references to appease you, but as soon as i'm back at my house i'll gladly read you a bed time story

i can assure you that unless you are damaging/removing a damage part of coax cable, the SWR will not appreciable change by haveing a different length of coax on a good properly built antenna mounted in accordance with good radio and engineering practices (not duck-tape to the spare tire etc). if your antenna is damaged or the cable is damaged its something else.....

im done kicking this horse...i think its dead
PM me if you wanna chat by phone on the topic

my radios work, and I dont have an even multiple wave length, i get my SWRs down by tuning an antenna

i hope you take me up on the phone offer...theres a few points you make i would gladly get into....

I'm done with this thread

 

Re: No ground plane antenna needed for fiberglass

ok, one more, becasue i hadnt read RRich's post

[qoute]Sooo - you could say in some cases coax length is not critical, in others it is - depending on HOW IT'S GROUNDED[/qoute]

YES, im not sure if this point was made before somewhere, but this is of the essence. w/o a good ground you get all sorts of goofy things going on

done...for real this time

 

Re: No ground plane antenna needed for fiberglass

[ QUOTE ]
ok, one more, becasue i hadnt read RRich's post

[qoute]Sooo - you could say in some cases coax length is not critical, in others it is - depending on HOW IT'S GROUNDED[/qoute]

YES, im not sure if this point was made before somewhere, but this is of the essence. w/o a good ground you get all sorts of goofy things going on

done...for real this time

[/ QUOTE ]

Once again, you've managed to read only bits and pieces and take them out of context in a desperate and feeble attempt to support your opinion. Same as you did on the receiver as a load, if not, then why did the next section start with the header The Antenna as a Load?

[ QUOTE ]
… depending on HOW IT'S GROUNDED

[/ QUOTE ]

NOT IF IT'S GROUNDED. As in grounded at the antenna end of the coax or only grounded at the transceiver end of the coax. He DIDN'T say ungrounded or poorly grounded but HOW IT'S GROUNDED.

Prior to that he said;

[ QUOTE ]
IF - IF - the coax is grounded at the antenna end, it must follow the above - the coax has to be an even multiple of the full wave. …

[/ QUOTE ]

So in the case where it is grounded and grounded at the antenna end, he says, "…the coax has to be an even multiple of the full wave." No cutting the coax.

But unlike your book The Radio Amateur's Handbook, which we don't know who the author is, we do know who the author of the reference you're now using is. Wanna ask him?

It's time you quit flogging this dead horse and for you to be the one to say, "I stand corrected."

It's a dead issue. It's over. I never was that interested while we were discussing it, I surely don't want to take my time to talk about it further via PM or over the phone. And especially not with someone who won't give up after Dawg posted a link to a guide by Cobra, a manufacturer of CB radios, which says CUTTING THE COAX CAN AFFECT SWR.

Give it up. /ubbthreads/images/graemlins/lame.jpg /ubbthreads/images/graemlins/30.gif /ubbthreads/images/graemlins/violin.gif /ubbthreads/images/graemlins/violin.gif /ubbthreads/images/graemlins/violin.gif

 

Re: No ground plane antenna needed for fiberglass

He He -- I was going to drop it too --- but I just remembered an interesting experiment the prof did when I was in college.

He took a xmtr into a SWR bridge/RF power meter, into a hunk of coax - I think it was a multiple of the wavelength, but not sure. Instead of an antenna he used a short piece of wire into a light bulb, then back to the shield. A purely resistive load.
SWR was really high. So he put a few turns of wire around the light bulb and shorted the ends together. SWR got much lower. Those shorted turns were inductive loads. They absorbed the radiated energy as a load too.

He kept on trimming and shorting those turns until he got as low an SWR as I've ever seen. The meter read almost 1:1 - the theoretical ideal point.

Thing is - the xmtr was putting out it's full output - going as strong as it could - it "saw" the proper load it needed to achive full power transfer -- but where was it going?

How much do you think was actually radiated into the air to be useful for communication? The signal strength meter across the room barely registered.

He was demonstrating how SWR and power transfer are related, and how actual radiation is not.

It's a very similar situation as the Firesticks and equiv use to "tune" their antennas with the slug.

I'm gone.

 

Re: No ground plane antenna needed for fiberglass

Thats why they call it FM......F*&%ing Magic....now, where is that beer?

 

Re: No ground plane antenna needed for fiberglass

FYI for anyone else interested in the topic,

I'll bring a few of my books here to the bash, lots of really cool stuff me thinks