One way of dealing with 15 is to just use an antenna tuner which basically
reuses the 40m dipole as a 3/2 wavelength antenna. You then just do
40/20/10 and have only three lines to worry about. You don't even have to
tune 15 separately. I have done this and it works well. It simplifies
construction, tuning, and deployment. Pattern is a little different on 15,
but I have found it effective.
Larry WO7R
On Sun, Apr 26, 2015 at 2:29 PM, Steve, W3AHL <w3ahl@att.net> wrote:
> Parallel multi-band dipoles require some attention to detail during
> construction and tuning, but they certainly are a good solution for many
> sites. I have hosted several antenna building parties for our local club
> where we have successfully completed almost 30 antennas, in a wide variety
> of band configurations and wire types. Four bands are the most I
> recommend, but we have built several five bands antennas that tuned fine.
> Many were built by new hams that hadn’t gotten on HF yet, due to the lack
> of an antenna. After the antenna is tuned, another mini-party is usually
> scheduled to help get it up in the air at owners site.
>
> All will cover the entire band (for 40-10M) at less than about 2.5:1 VSWR,
> except on 80M, assuming a reasonable height (40-70+ ft.). We try to tune
> them at the height and vertex angle for the final site, but sometimes a
> little tweaking is needed at the final installation site, especially on 15
> and 10M when mounted as an inverted V. Tuning is done with an AIM-470C,
> allowing the owner to have a record of what the tuning was for future
> reference. I usually include an EZNEC plot for azimuth and elevation
> patterns and sometimes an HFTA example of how their site affects the
> pattern in key directions.
>
> Tuning is fairly straightforward if the antenna is built correctly and
> measured accurately. Start with the longest elements first and work your
> way down. Tuning the shorter elements will not affect the longer ones.
> One problem to be aware of is the interaction between 40 and 15M elements,
> especially if 40M is tuned a little high to favor the upper part of the
> band. It can make tuning 15M very frustrating, until 40M is tuned just a
> little lower. Yes, you can operate 15M using the 40M element at its third
> harmonic if you use a tuner and don’t mind the nulls in the pattern.
>
> They make a good club project, since it really helps to have someone on
> the team that has built and tuned one before! The most critical part is
> getting the wires dressed correctly where they fan out from the feed
> point. The hardest part is getting people to follow the directions
> carefully and measuring the wire accurately so the two sides stay balanced
> as they are tuned. It is hard to build one by cutting the wires to length
> from a table and expecting it to centered in the desired band. I find
> that 6” spacing between elements work well. A four-band 75/40/20/10
> antenna can be built for less than $40, plus feed line and supporting
> ropes, pulleys, etc. It takes 4-8 hours to build and tune (with 2-3 people
> helping for some parts of the project) depending on experience level and
> how much time everyone spends chatting (antenna parties tend to be social
> events!).
>
> While just about any piece of wire can be made to work to many operators’
> satisfaction, dipoles always seem to perform more consistently, produce
> less RFI in the shack and can run the legal limit without a tuner in most
> cases, at least for me.
>
> Steve, W3AHL
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