[TowerTalk] Looking for a Short Rotatable Dipole or...

[Roger (K8RI) on TT]

As Jim and Jim have both stated, a low dipole has a high angle of 
radiation and no real lobes. Two phased at that height might give you 
some f/b, but the directive would be questionable.  The phasing would 
allow you to maximize the gain and front to back. likely the gain would 
be small, but the f/b might be useful.  OTOH the two dipoles are likely 
to have more of an effect on the 15 meter beam than a single dipole.

With a small lot it take some imagination, ingenuity, and time to work 
through trial and error  to get multiple antennas to work without 
trashing each other, particularly, on harmonically related bands (third 
harmonic)  such as 40 and 15 .  IE. you can use a 40 meter ant on 15.

You might consider a multiband vertical like the Hy-Gain AV640.  I have 
one mounted on a mast at about 25 feet.  Many times it matched or 
exceeded the sloping dipoles

On a small lot I don't know if you have room for a sloping dipole, let 
alone several, but even one or two at 30 up and 30 out might give you 
the coverage you desire and would "likely" have less effect on the 15 
meter beam. Depending on the size of the beam it could have a 
substantial effect on the 40 meter dipole

A sloping half wave dipole  on 40 with the top end at 30 feet with the 
bottom only 30 feet out gives you 44 fee for the dipole. With a 40 meter 
dipole being roughly 33 feet long that gives you 9 feet of clearance off 
the bottom end.  That would put the bottom end about 4' above ground 40 
feet out would give you another foot or so of clearance.  at 30 up and 
30 out, you'd need a 60' diameter circle for the 5 element sloper.

With a lower angle of radiation, some gain, and a f/b unavailable for 
the level dipole, or even a phased pair.  That height would also give 
you some coverage of both.

The switched,  5 sloping dipoles as shown in the handbooks would likely 
give you more of both The cost would be for the remote antenna switch, 
wire, center insulators, and additional coax. Unfortunately I doubt you 
have the room.
Most of us are short of something

73

Roger (K8RI)




On 4/29/2016 Friday 6:30 AM, jimlux wrote:
> On 4/28/16 5:49 PM, Roger (K8RI) on TT wrote:
>> Just another thought:
>>
>>  30' pole on the roof supporting 5 center fed, half wave sloping
>> dipoles. I have used a couple, but if you have the room, use the 5
>> switched dipoles.  I believe it's in either the ARRL handbook, or the
>> Antenna handbook..  They are simple, seem to work stateside and DX.
>> Switched as in the handbook, the have a little gain and a bit of a F/B
>> that really helps.
>>
>> Even 2 or three will give a bit of directivity.    I'm thinking of
>> switching to 75/40 fan dipoles and experiment with the 5 antennas
>> instead of just a single monoband on 75 and one on 40 at present. Wire
>> is cheap and easy to experiment with.  I've worked all continents with
>> just two 40 meter slopers and an AV640 (on 40) at 25 feet.
>
>
> I would think that two dipoles, with some sort of adjustable phase 
> combining network might be the optimum strategy.  A dipole isn't going 
> to have good directivity for the main lobe, but can have a pretty good 
> null to place on an interferer.
>
> There was a clever scheme in one of the antenna compendiums where 
> there was a adjustable LC, a combiner, and a DPDT polarity reversing 
> switch.
>
> You adjust the two knobs on the LC to minimize or maximize the signal 
> as desired, flip the switch and what was maximized, is now minimized.
>
>
>
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-- 

73

Roger (K8RI)


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