[TowerTalk] [Bulk] Re: 40-30m dipole design - GOALS

[Grant Saviers]

375 ft of LDF5 feedline on 30m = 0.4db loss at 1:1 swr.  I you use the 
75ohm 7/8, even less loss.   So, if this was my design choice as a DXer, 
I would optimize for 40m with a full size dipole and take the SWR loss 
hit on 30m by using a tuner.  If that is too much wind load then a 
shortened 40m dipole with high Q loading coils at 40% out +/-.

You don't mention separate receive antennas, but noise generally should 
be less with the dipole vs V's.  Use EZNEC to figure out how much you 
might gain at low angles with the dipole at 105' vs the existing V's.  
Compare that to HFTA arrival stats for your QTH. That is what will make 
the difference for DX, plus a bit of front to side.  Compare the RDF of 
the two.

Then decide if it is worth the effort and $ per db.

Grant KZ1W



On 3/18/2016 5:49 AM, N1BUG wrote:
> On 03/18/2016 06:27 AM, Tod wrote:
>> Paul, what is the point of this speculation? A single antenna
>> that covers two bands? A rotary antenna for 40 and 30?
>
> Both. I should have specified my goals.
>
> I am a DXer first, contester second. Currently I have a 40 meter 
> inverted V, apex at 70 feet and 30 meter inverted V, apex at 80 feet. 
> Here is what I would like to achieve:
>
> 1. Get the 40 and 30 meter antennas farther from homes and multiple 
> small sources of RFI which have already been mitigated to the extent 
> reasonably possible. Some quick tests with inverted V and sloper 
> antennas in the proposed location would seem to indicate it is quieter.
>
> 2. Upgrade to higher performance antennas if possible, but definitely 
> not lose DX performance compared to what I have now. The proposed 
> rotatable dipole would be at 105 feet minimum.
>
> 3. Reduce the number of ropes tied off around the property by going to 
> non-wire antennas on these two bands. I'm the one who has to mow the 
> yard and would like get rid of some tie-off posts.
>
> 4. Keep band switching to a minimum. I didn't like my own proposal in 
> my first post on this criteria, but I have been struggling with 
> less-than-ideal options for weeks.
>
> The constraints are:
>
> 1. Budget. If I weren't on a budget, I would already have multiple 
> towers with stacks for every band. :)
>
> 2. I need to watch how much load I add to the rotator and mast.
>
>> Get someone  to model for you a dipole, center fed,  that has the
>> length you can put in place.
>
> I could do a full size 40 meter dipole. This was my original plan 
> before deciding I wanted to see if 30 meters could be included. I 
> still may end up going back to this.
>
> I thought about full size parallel dipoles for 40 and 30 but the 
> amount of aluminum became a concern for rotator and mast stresses.
>
>> pick a feedline that 'suits' you
>
> A 375 foot run of LDF5-50A 'suits' me -- mostly because it is already 
> paid for and installed! I had a lot of fun snaking it up the inside of 
> the tower (my preference) working alone. :)
>
> There is also a 375 foot run of 3/4" 75 ohm cable that could be used.
>
>> 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 .
>
> That could be acceptable if the antenna only requires a tuner on 30 
> meters, since a low power tuner would suffice and band switching could 
> be as simple as bypassing the tuner. I would prefer not to get into 
> another set of knobs to be twiddled/set when changing bands.
>
>> 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.
>
> Effort is of no concern. I am used to working harder to achieve 
> satisfactory results on a budget. :)
>
>> 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 what I have been reading is to be believed, going from inverted V 
> to dipole and 70/80 feet to 105+ feet should be an upgrade even with 3 
> dB loss.
>
> 73,
> Paul
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