dipole question
 

Hi Group,

Let's say I have a 15 meter dipole. If I were to take a ferrite core
and place a core along the dipole for the lenght of 6 meters, having
two cores on either side.

The question is would this antenna now work as a dipole on 6 meters but
not on 15 unless the reactance of the core was low at 21 MHz?

Does the core act as a RF block but not as a inductance with a
reactance at 15meters?

If it is a inductive reactance, then reducting the lenght of the dipole
would bring down swr on 15m.

I have seen the ferrite emi cores which clamp around the wire, would
these work for 100 watts?

If I had a way to move the cores along the dipole, I could create a
multiband antenna, true?

I am sure there are better ways to do such, aka trap dipole but I like
to give this some thought.

De KJ4UO

The short answer is no. One of the best readily-available ferrites for
VHF use is Fair-Rite type 61. At 6 meters, one core of this material
will look like a resistor of a few ohms in parallel with an inductor
whose reactance is also a few ohms. That combination won't do what
you're asking.

Even if you locate a better material, you'll still only have a few ohms
of reactance for a single core at that frequency, and the Q is likely to
still be not very good. It'll be a long way from making a trap, which is
what you're trying to do.

Roy Lewallen, W7EL

wrote:
Hi Group,

Let's say I have a 15 meter dipole. If I were to take a ferrite core
and place a core along the dipole for the lenght of 6 meters, having
two cores on either side.

The question is would this antenna now work as a dipole on 6 meters but
not on 15 unless the reactance of the core was low at 21 MHz?

Does the core act as a RF block but not as a inductance with a
reactance at 15meters?

If it is a inductive reactance, then reducting the lenght of the dipole
would bring down swr on 15m.

I have seen the ferrite emi cores which clamp around the wire, would
these work for 100 watts?

If I had a way to move the cores along the dipole, I could create a
multiband antenna, true?

I am sure there are better ways to do such, aka trap dipole but I like
to give this some thought.

De KJ4UO

On 5 Jul 2005 20:20:23 -0700, "
wrote:

Hi Group,

Let's say I have a 15 meter dipole. If I were to take a ferrite core
and place a core along the dipole for the lenght of 6 meters, having
two cores on either side.

The question is would this antenna now work as a dipole on 6 meters but
not on 15 unless the reactance of the core was low at 21 MHz?

Does the core act as a RF block but not as a inductance with a
reactance at 15meters?

If it is a inductive reactance, then reducting the lenght of the dipole
would bring down swr on 15m.

I have seen the ferrite emi cores which clamp around the wire, would
these work for 100 watts?

If I had a way to move the cores along the dipole, I could create a
multiband antenna, true?

I am sure there are better ways to do such, aka trap dipole but I like
to give this some thought.

De KJ4UO

If you want to have a single antenna for 15 meters and 6 meters, you
may find it easiest and more efficient to do what I did with my 20
meter dipole. I cut a 6 meter dipole and hung it under my 20 meter
dipole. I happened to cut the length perfect for 50.125, which is
about where I wanted it. The 20 meter dipole resonance dropped.
however, it was a bit on the high side, a perfect match for 14.336
before hanging the 6 meter dipole. now it is a bit lower, but it is
broad banded enough to cover the entire band at 1.3:1 or less so I
didn't need to trim it.

The two dipoles come together at the coax connector and an 8" piece of
PVC pipe is used as an insulator and hangs from the 20 meter dipole to
the end of the 6 meter dipole to keep the ends away from the 20 meter
dipole.

I have been making and using antennas like this since I was a novice.
The only thing I have ever noticed different from single dipoles is
that the lower bands are more broad-banded on the parallel dipoles.

BTW, the six meter dipole is great. It doesn't compare to my
neighbors with beams, but it is better than nothing, and it is a
dipole.


--
73 for now
Buck
N4PGW

On Wed, 06 Jul 2005 07:47:55 -0400, Buck wrote:
On 5 Jul 2005 20:20:23 -0700, "
wrote:

Hi Group,

Let's say I have a 15 meter dipole. If I were to take a ferrite core
and place a core along the dipole for the lenght of 6 meters, having
two cores on either side.

The question is would this antenna now work as a dipole on 6 meters but
not on 15 unless the reactance of the core was low at 21 MHz?

snip

If you want to have a single antenna for 15 meters and 6 meters, you
may find it easiest and more efficient to do what I did with my 20
meter dipole. I cut a 6 meter dipole and hung it under my 20 meter
dipole.

snip II

A dipole cut for the low end of 40M should work "OK" (for
selected definitions of "OK") on both 15M and 6M, as well.

I often work fellas on 6M who apologize for "using a 40M dipole."
Their sigs are usually every bit as strong as others.

73
Jonesy
--
Marvin L Jones | jonz | W3DHJ | linux
Pueblo, Colorado | @ | Jonesy | OS/2 __
| config.com | DM78rf | SK

Allodoxaphobia wrote:
I often work fellas on 6M who apologize for "using a 40M dipole."
Their sigs are usually every bit as strong as others.

For a 40m dipole running North/South, the major lobes on 6m
would be N27W, N27E, S27W, S27E, i.e. pretty much end fire.
I use my 20m rotatable dipole as an emergency TV antenna
when cable is down.
--
73, Cecil http://www.qsl.net/w5dxp

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