yet another dipole question......

[oldslowchevy]

if i set my dipole for 11 meter for 27.185 each side has to be 8' 7-5/16" long, 17' 2-5/8" over all.

now can i shorten the over all length by coiling the leads around some pvc or a wooden dowel as long as i keep each wire at 8' 7-5/16" long, even though the whole antenna may only be 8-10' long by coiling the leads.

i really hope you understand what i mean.

 

[Captain Kilowatt]

You can but it will not be tuned to 27.185 any more. You cannot simply take a tuned antenna and wind part of the elements into a coil to shorten the overall length without drastically affecting the tuning. The entire antenna will need to be retuned and the impedance at resonance will no longer be 50 ohms but rather something less. The efficiency of such an antenna will also be considerably lower than a full sized antenna.

 

[oldslowchevy]

ahhhh, all part of the learning curve.

the reason behing the question was like many cb antenna that have the "load coil" like wilson and k-40 as well as many others have, were designed to shorten the antenna.

i was thinking (not always a good idea) that in theroy it could work.

 

[Robb]

If you were thinking of a choke coil; the dipole wires is NOT where it goes.

The choke coil is used on the top end of the coax - just below where the dipole wires are connected to the coax.

A choke coil for the 11m/CB band requires 5 turns of coax tightly wound together on a 4.25 inch diameter coil form.

It is just that simple - yes.

 

[hookedon6]

...the reason behing the question was like many cb antenna that have the "load coil" like wilson and k-40 as well as many others have, were designed to shorten the antenna..

actually, a coil makes the overall antenna longer and resonant at a lower freq,...... the
"stinger" may be shorter, but the overall antenna is electrically longer

 

[oldslowchevy]

If you were thinking of a choke coil; the dipole wires is NOT where it goes.

The choke coil is used on the top end of the coax - just below where the dipole wires are connected to the coax.

A choke coil for the 11m/CB band requires 5 turns of coax tightly wound together on a 4 1/4 inch coil form.

It is just that simple - yes.

no not as a choke just tring to make it more "compact" i had the thought of standing it on end (floor to ceiling) but only have 8' to work with.

just having ideas going though my head.

my balun is abit different then most as i built it years ago before i have the knowage i have now. i have a 12" long 2 1/2" pvc pipe with 18' of coax coiled inside of it with couplers mounted on each end in the end caps. i have been told by many that it should not work, then they are amazed that it works better then the ones they have made by using the right theroy. chalk one up for over kill.

 

[Robb]

no not as a choke just tring to make it more "compact" i had the thought of standing it on end (floor to ceiling) but only have 8' to work with.

just having ideas going though my head.

my balun is abit different then most as i built it years ago before i have the knowage i have now. i have a 12" long 2 1/2" pvc pipe with 18' of coax coiled inside of it with couplers mounted on each end in the end caps. i have been told by many that it should not work, then they are amazed that it works better then the ones they have made by using the right theroy. chalk one up for over kill.

First of all, let's break down how a choke coil works.

A choke coil is a tuned circuit. It will work because it offers resistance to common mode current on the coax at a particular frequency. This resistance keeps this 'current' from flowing on the outside shield twards the radio. In particular, the 27mhz/CB band.

We could make another choke for one of the Ham bands freqs; but this would require more coax winds in the choke in order to tune it for that wavelength/freq. Even the diameter of these chokes will have an effect on how much resistance we can squeeze out of them for any one freq. We need to get all of the resistance we can and where we need it to be.

More turns won't make more resistance at our desired freq. In fact, it may well have far less resistance. Because it will have tuned it to have more resistance at a lower frequency than the one that you need!

Using a narrow coil form - as you claim you are doing - is not going to offer the amount of resistance to common mode current that you seek. In truth, it will have less. Chokes use formulas and need to be used - word for word - to reach the goal of maximum resistance to common mode current at any given frquency.

So; your choke may be working. But I can assure you that it could be better that what you have now. These things that we use didn't just fall off of a turnip truck; they are arrived at by/thru scientific observation. Which means, that the results are consistent and repeatable.

I suggest that you try to 'repeat' what is known to work.
These results can only be better for you.

As a side note, resistance is a term used for DC circuits. With AC or RF output circuits, it is called 'impedance'. So, what you really doing is changing the impedance to the maximum. That is; if you follow the formula and build your choke for the CB band - as you should for best results.

5 tuns of tightly wound coax on the outside of a 4.25 inch diameter coil form.
No more; no less . . .

http://www.worldwidedx.com/cb-antennas/48415-ugly-balun.html

 

[Beetle]

I agree with Robb, except that you should substitute the word "impedance" for "resistance" wherever it appears.

The impedance a choke offers is only to RF on the outside of the shield; since the inside of the shield remains the same distance from the center conductor, the number of turns doesn't affect the RF inside the shield.

Be very careful not to wind the choke's turns too tightly. Heat and age can cause the center conductor to migrate through some types of coax. Generally I recommend 8" diameter for the 0.405" O.D. coax and 5" for the smaller types.

 

[Robb]

You are right, Beetle. I should have been a bit more specific in a few places.

1) The coil form must not be metal. It has to be plastic, cardboard tube, etc (non-conductive material).

2) The coil form needs to be 4.25 inch diameter for RG-58 coax, and 8 inch diameter for RG-8 coax.

3) If using a choke on an antenna mounted on a metal mast; make certain that the choke isn't touching the metal mast. Metal will detune the choke, and we don't want that. You will need to use a spacer to keep the choke 5-6 inches away from the mast.

 

[oldslowchevy]

robb, as i stated i made this one looooooooong time before i knew of or had ready access for the correct formalas. butven with access i am still learning but much is still lost on me which is why i come here or other places to learn and ask questions.

like i said though it works, by how much i don't know. :redface:

 

[Captain Kilowatt]

I disagree on a point or two Robb. The choke coil is not nearly as much of a tuned circuit as you think. If it were then the typical tribander with split element feed would not have an effective choke since it must cover from 14.0-29.7 MHz. It is the presence of inductance alone that chokes off the RF and not a tuned LC circuit. Also placement of the choke in close proximity to a metal object will not detune it either as it is not tuned in the first place. In fact Cushcraft RECOMMENDS taping the choke to the support mast.The following is a cut and paste from the Cushcraft A3W tribander assembly manual.

"#4 - RF CHOKE
A feedline should be prepared as shown in Figure H using the
solder terminals (453) provided. A 1:1 balun may be installed
at the feedpoint but it is not required. We recommend using
an RF choke made from 8 turns of RG8/U coaxial cable with
6 inch (15.3 cm) diameter as shown here. Important- Do not
use foam dielectric coax for your RF Choke because when the
coax is wound in a coil the center conductor may migrate away
from the center and detune your choke. Attach the RF choke
to the driven element using washers (41) and nuts (11). Be
sure to tape and seal the feedline against water at the point
where the center dielectric and braid separate. Tape the
feedline, between the feedpoint and the RF choke, to the boom.
Tape the RF choke to the mast. If you plan to install the antenna
in a salty or corrosive environment, you may want to consider
coating all elements with a clear marine varnish or its equivalent
after it is assembled."

The reference to detuning above is in reference to altering the impedance of the cable and not to detuning as in the normal sense regarding frequency of operation. Most guys orient the plain of the loop of the coil parallel to the boom and tape thru the coil and around the boom with no ill effects at all.

 

[Robb]

You are having a problem with your dipole. Yes. We now know that you have shoved coax into a 2 in diameter tube. You may have kinked the coax in the process; kinked coax can affect its 50 ohm status. This can have an adverse affect on the 'impedance' too.

So can the power wires in the walls inside of your apartment. Some power wires are horizontal and some are vertical. But it is proximity to these wires - and other large metal objects in the room that can affect and change your SWR too.

 

[Robb]

CK:

I used the following chart as a reference for choke impedance curves.

Everything I've read on chokes suggest keeping metal away. That is what I based it on.

One can also see on http://www.worldwidedx.com/cb-antennas/48415-ugly-balun.html page that there is no metal used for coil forms nor has anyone used metal in close proximity either. Save for ferrites; which is a different animal.

If you look at the green bars ('highst impedance') on the chart (above) for 27mhz, you will see that the info for the air-cored chokes is correct.


convenience or necessity?

 

[HomerBB]

My name is Homer, and I agree with this message.

 

[oldslowchevy]

You are having a problem with your dipole. Yes. We now know that you have shoved coax into a 2 in diameter tube. You may have kinked the coax in the process; kinked coax can affect its 50 ohm status. This can have an adverse affect on the 'impedance' too.

So can the power wires in the walls inside of your apartment. Some power wires are horizontal and some are vertical. But it is proximity to these wires - and other large metal objects in the room that can affect and change your SWR too.

robb i detect a bit of either anger or fustation. please know i am not tring to be a know it all or a smart butt.

ok that said, i did have a dipole that i was having isuses with (found the problem) but i have been using my .... balun, filter, whatever it is for 3 years with zero isuses. the coax took along time to coil because i did take great care to coil it carefully not to kink or overlap any of the windings.

the wall that the dipole is on has one light switch and one receptical that my power supply is plugged into. this set up has not changed other then swapping radios in and out in years.

the problem with the my first antenna was the wire was broken 18 inches from the solder joint. i used what i had laying around when i made it. some thin thnn soild core wire.

i made my new one using the same coax and some 4 gauge wire i used for a power inverter in my old 379 pete

swr 1.2:1:w00t: and all is right with the world again

sorry for any aggervation i may have caused you because i really wasn't tring to give you any.