[TowerTalk] Fwd: Bandwidth of a 75/80 meter dipole

[Mirko S57AD]

I'm affraid that quarterwave 75 Ohm impedance transformer is excellent 
approach for dipole(s) and/or single vertical, but quite useless for 
broadening bandwith of 4 square array.

73 Mirko, S57AD

Pete Smith pravi:
> It's even better than that - a piece of 75-ohm coax permanently in the line 
> at the right place gives a frequency characteristic that has a 
> double-dip.  I think the idea was first thought of (or at least publicized 
> in amateur circles) by W6NL.  Here's a reference, from 
> N3BB 
> <www.ctdxcc.org/n3bb/broadband_antenna/A_Broadband_80_Meter_Antenna_System.ppt  
>  >
>
> 73, Pete N4ZR
>
> At 04:13 PM 3/17/2009, W0MU Mike Fatchett wrote:
>   
>> K5TR has another way.  He sent me how to do it but I can't find it.  I
>> recall a piece of 75 ohm coax that was switched into the line.
>>
>> Maybe Geo will post it.
>>
>> Mike W0MU
>>
>>
>> CC Packet Cluster W0MU-1
>> W0MU.NET or  67.40.148.194
>>
>> "A slip of the foot you may soon recover, but a slip of the tongue you may
>> never get over." Ben Franklin
>>
>>
>>
>> -----Original Message-----
>> From: towertalk-bounces at contesting.com
>> [mailto:towertalk-bounces at contesting.com] On Behalf Of K4SAV
>> Sent: Tuesday, March 17, 2009 8:55 AM
>> To: towertalk at contesting.com
>> Subject: Re: [TowerTalk] Fwd: Bandwidth of a 75/80 meter dipole
>>
>> There are several ways to make a dipole cover all of 75/80 meters, and also
>> many ways that don't work.
>>
>> A double Bazooka doesn't cut it.  It only provides about 14% increase in
>> bandwidth at the expense of extra loss at the band ends.  Even the crossed
>> double bazooka that provides about 55% increase in bandwidth at the expense
>> of even more loss, doesn't do it.
>>
>> A cage dipole doesn't do it either unless you make the cage an extremely
>> large diameter.
>>
>> A folded dipole will provide a little more bandwidth but it won't cover the
>> whole band either.
>>
>> A fan dipole with one dipole cut for CW and one cut for the phone end will
>> work if you place the dipoles at right angles to each other.
>>
>> This open-sleeve version of a dipole will cover all of 75/80 with less than
>> 2 to 1 SWR and have good efficiency.
>> http://rudys.typepad.com/ant/files/antenna_broadband_dipole.pdf
>> Be careful if you try to analyze this antenna with NEC.  It is possible but
>> extremely tricky to analyze.
>>
>> The method of applying a resonant LC network at the feedpoint of a dipole,
>> as shown in Chapter 9 of the ARRL Antenna Book will work, however the
>> version with the whole LC network connected directly across the antenna will
>> be limited to low power unless you use some honker size caps to handle the
>> current.  The method used in the "DXers Delight"
>> (same chapter) can be used at high power.  It provides a step-up impedance
>> for the caps so they don't have so much current.  Neither of these work well
>> at high power using a toroid for the coil because of the huge flux values.
>> This technique produces three resonant points in the 75/80 meter band
>> although resulting in an increase in feedpoint impedance which then has to
>> be matched.  It is possible to implement all the whole network and the
>> matching with a single tapped coil.
>>
>> There are many other ways to do this that also work with good efficiency.
>> Some of them are shown in Chapter 9 of the ARRL Antenna Book.  Beware of
>> networks using coax in the matching network.  Many of those (but not all)
>> are very inefficient.
>>
>> Jerry, K4SAV
>>
>> _______________________________________________
>>
>>
>>
>> _______________________________________________
>> TowerTalk mailing list
>> TowerTalk at contesting.com
>> http://lists.contesting.com/mailman/listinfo/towertalk
>>
>> _______________________________________________
>>
>>
>>
>> _______________________________________________
>> TowerTalk mailing list
>> TowerTalk at contesting.com
>> http://lists.contesting.com/mailman/listinfo/towertalk
>>     
>
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
>