# Shack ideas

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KE0CPH, Oct 17, 2018.

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1. ### KE0CPHHam MemberQRZ Page

I currently don't have a shack setup but I've got some plans to hopefully be up on HF before spring.

I'm going to be using 450 ohm ladder line up to a doublet measured for 3.5 MHz, so about 67 feet on each leg. I'll be receiving a MFJ 949E tuner to go along with my FT-101ZD. I was a bit curious if I could use the doublet for 2 meters. Now I know that the MFJ only goes to 30 MHz but if I were to find a tuner for high frequencies, how well do you guys think that'd work?

I do have a spare 2 meter radio, a TM-181A, I'd just need to find a suitable PSU for it.

I was reading on the amateur radio subreddit that you should make sure that the spacing of the two conductors is less than 1% of the wavelength you're working on, so that'd be 20 cm for 2 meters. Any truth to that?

2. ### K3UJHam MemberQRZ Page

Don't for a few reasons.
Your doublet is horizontal and the majority of 2M FM is vertical.
For the cost of a 2M tuner that will accept balanced line, you can get a proper antenna with coax.

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3. ### AI3VHam MemberQRZ Page

What happens is as the dipole gets longer in WAVELENGTHS the radiation pattern changes from the classic figure eight of a half wave dipole, and starts to develop lobes and nulls.

If the desired station is in the direction of a lobe, great, if it's in a null, not so great.

A rhombic antenna is just a special shape of a many wavelength antenna that takes advantage of this effect to provide a very high gain beam in a specific direction.

Read up about rhombic, half rhombic, and the extremely common "vee beam" used on rooftop vhf tv antennas.

Short answer, you will have a terrific signal, mostly horizontally polarized,in some directions.

Mr Murphy will ensure that nobody is in that direction you want to talk to.

Rege

P.s.
On a radio path, unless you have a true optical line of sight path, but instead a path that has reflections and refractions between stations, you will receive a mixture of polaritys.

Fm radio stations all most allways transmitter circular polarization to counter this effect.

It is also used in some radar sets (like the illuminator for a sea sparrow missle) the transmitter and receiving antennas are at right angles to one another.

I would not use a "tuner", but I would wind a small torodial impedance matching balun.

Last edited: Oct 17, 2018
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4. ### KE0CPHHam MemberQRZ Page

Wow, talk about terrible math though. 1% wavelength for 2 meters is 2 cm, not 20 cm lol.

5. ### AE8WHam MemberQRZ Page

I sort of share the curiosity about how well it will work. But given that there are no givens of expectation I don't think it is going out on a limb to say that it will be perfect for what you want. LOL

On the other hand, if you stuck up even a 1/4 wave whip with 4 radials it will probably be better in all respects relative to the 80m doublet OR it might work just as well depending on that expectation.

Anyway, I am far too lazy to do any analysis or modeling. As a result, I have no idea if the impedance at 2m can be tuned by some tuner. RF in the shack seems like a good bet as I am sure RF will radiate somewhere regardless of the match.

6. ### KE0CPHHam MemberQRZ Page

Well, I got to thinking. What if I made something like a fan doublet where I just have the ~140ft part and connect up a 2 meter doublet on the same ladder line. I'd have it slanted, not perfectly up and down to avoid interference with the ladder line.

So the HF doublet would come out of the ladder line at a 90° while my 2 meter doublet would be at a 45°.

I've attached an image to visualize what I'm talking about.

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7. ### WB2WIKPlatinum SubscriberPlatinum SubscriberQRZ Page

Why would you even want to do that?

It's likely to work really poorly.

For FM/repeater work, everything is vertically polarized and a \$5 ground plane will run rings around what you've proposed. For SSB/weak signal work, everything is horizontally polarized and you'd need to rotate the antenna unless you only want to work stations in one or two directions and ignore the rest of the map.

There's an old saying, "When something is both possible and obvious, and nobody's doing it, there's a good reason for that."