[TowerTalk] 40-30m dipole design

[Tod]

Paul, what is the point of this speculation? A single antenna that covers two bands? A rotary antenna for 40 and 30? 

Get someone  to model for you a dipole, center fed,  that has the length you can put in place. Find from the model how it will radiate on 40 & 30 if you can feed power to the antenna. Find the feed point impedance and pick a feedline that 'suits' you --- almost certainly you will end the modeling exercise with standing waves on the feedline. 

Feed the station end of feedline using an antenna tuner so your transmitter will deliver power to the feedline ( and as a result to the dipole).  If the SWR on the feedline is 10:1 or less quit spending time on the design and go ahead and build and use the antenna. If you decide to use open wire line for all or part of the feedline from the station to the antenna don't worry about feedline losses even if they are greater than 10:1 . 

I will leave to others to reassure you that the power will go out to the antenna and, while some may be lost in the feedline, the rest of the power will go into the antenna and be radiated. Do not bother with traps, center mid-point or end. The objective is not to make a resonant antenna, but rather a dipole which is resonant at some frequently, not necessarily the one you plan to transmit on. Getting close to the frequency you want to transmit on is good, but not a requirement. 

What I have written above does not produce an optimum antenna. It should produce one that will work satisfactorily. It also should take a lot less effort to create and install. 

Careful examination of the difference between optimum and this sub-optimum version in terms of radiated power will show that the degradation from optimum is not really significant in terms of on the air results. 

Even if only half the power were radiated, the signal will be only 3 dB lower than if all were radiated. For almost all operating this will be of no consequence. If it is of consequence you have a completely different objective than I perceived while reading your initial email.

Tod, K0TO







Sent from my iPad air


> On Mar 17, 2016, at 4:09 PM, jimlux <jimlux at earthlink.net> wrote:
> 
>> On 3/17/16 2:54 PM, jimlux wrote:
>>> On 3/17/16 12:40 PM, Jim Brown wrote:
>>>> On Thu,3/17/2016 12:14 PM, jimlux wrote:
>>>> There's no big advantage to linear loading: you might as well use a
>>>> good low loss inductor at the feed (the "shorty 40" does this).
>>> 
>>> Are you certain about this, Jim? Both change the current distribution,
>>> but linear loading changes it least at the center, where current is
>>> greatest. The inductor places maximum current in the inductor, which
>>> doesn't radiate.
>> 
>> I agree.. It's like base loading a vertical (with a top hat perhaps) vs
>> loading it half way up.  BUT, a 30m dipole isn't that much shorter than
>> a resonant 40m dipole, so the current distribution is nicely modeled by
>> the usual "half a sine wave from end to end".  That current distribution
>> doesn't have significantly different gain (1.93 dBi vs 2.14 dBi).
>> However, if a infinitely small dipole is 1.5dBi and a full size dipole
>> is 2.15dBi, the gain is going to be somewhere in the middle.
> 
> I just ran a NEC model (using 4NEC2 to compute the matching network).. the 15 meter long (30m band) dipole with a T network (91nH series, 352pF shunt, 7.47 uH series) has 1.75 dBi gain (vs 2.14dBi for ideal dipole).
> 
> The 2:1 bandwidth is a bit more than  300 kHz; 1.5:1 bandwidth is about 150 kHz.
> 
> I leave it as an exercise for the reader to figure out the voltages and currents on the components (which was the original question)
> 
> 
> So, in summary
> 
> full size 40m dipole: 2.15 dBi gain
> 75% size dipole: 1.93 dBi gain
> 75% size dipole with coil Q 250, cap Q 1000: 1.75 dBi gain   (about 80-84 degree 3dB beamwidth)
> 
> 
> the 40 m full size antenna has a swr of 1.4:1 at best and a 2: 1 BW of about 400 kHz (not much wider than the 75% sized antenna with a matching network).  And the match is better with the matching network (50 ohms vs 72 ohms.
> 
> The beamwidth of the full size is slightly narrower (76-80 deg)
> 
> 
> 
> _______________________________________________
> 
> 
> 
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk