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Author Topic: Dipole antenna seperation  (Read 1236 times)

Elf36

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Dipole antenna seperation
« on: October 20, 2022, 1426 UTC »
I am planning on installing my 135' OCF dipole at 40' and would like to put a 15M dipole below it at maybe 30'. I know that separation is important, but I obviously don't have room to separate a wavelength between the two. In your opinion, how close can they be together without causing issues? Thanks

Offline ThaDood

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Re: Dipole antenna seperation?
« Reply #1 on: October 20, 2022, 1756 UTC »
Quick answer, very close, if they are perpendicular to each other. Heck, if you are planning that route, you could try a multi-band Fan Dipole, and save on the number of coaxial cables. BTW, the 135ft OCF dipole may do a decent job on 15M. Passing thoughts, especially when dealing with limited space. What wasn´t mentioned is, are you panning to RX only, or RX / TX, with these?
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Re: Dipole antenna seperation
« Reply #2 on: October 20, 2022, 1844 UTC »
I would definitely recommend the fan dipole approach vise the separate antenna. The antenna underneath the OCF can act as a counterpoise or reflector and mess up the OCF pattern. This "random" wire is kind of the same approach which is us on AM towers. A tuned wire is added on some point on the tower, or an additional tower, which effectively suck the RF to ground in that direction creating a null in the pattern. This is a common method used for creating the day time / night time pattern.

I once built a folded end-fed antenna, same idea as a T2FD but end-fed vise being centre fed, and that antenna was a disaster. Separation between the wires was 18 inches. Needless to say, it did not last long. Half that antenna, 44 feet of it, is my current end-fed longwire. I had to remove the bottom half.

Have a look at some of the Palomar Engineers notes on OCF antennae:
https://palomar-engineers.com/tech-support/tech-topics/off-center-fed-ocf-antennas/Antenna-Systems-c21444176
https://palomar-engineers.com/tech-support/tech-topics/antenna-notes/off-center-fed-dipole-notes

It is no secret that I am a big fan of Palomar Engineers products. Some of their antenna products and accessories are manufactured for the military so you known you are getting the quality of parts, manufacture and support for your investment.

There are some KiwiSDRs running Palomar Engineers OCFs and so far I like the results. The antennae seem to be quieter and less noisy than others.

And yes, my end-fed is terminated with a Palomar Engineers bullet transformer and a common noise choke at the receiver side.

P.S. All of the Palomar Engineers OCF antennae covers 15 metres. The 80-10 meter antenna is 134 feet. If you have the realestate, the 160-6 metre one is 234 feet and if you have less realestate, the 40-10 metre one is only a little over 64 feet long.
« Last Edit: October 20, 2022, 1900 UTC by ~SIGINT~ »

Elf36

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Re: Dipole antenna seperation
« Reply #3 on: October 20, 2022, 1856 UTC »
Yes. I plan on TX & RX on both. The 135' OCF will not tune 15 or 30M using my FT-950 internal tuner. I forget at the moment what the SWR readings were when tested with a NanoVNA. Thanks for your feedback.

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Re: Dipole antenna seperation
« Reply #4 on: October 20, 2022, 1902 UTC »
Add a resonator stub as described here:
https://palomar-engineers.com/tech-support/tech-topics/antenna-notes/off-center-fed-dipole-notes

You will have to figure that out with some math and your VNA.

P.S. I just looked at the manual for the FT-950 and it states that if the SWR is greater than 3:1 that the internal ATU may not be able to reduce the impedance mismatch to an acceptable value. Sounds about right. My ICOM is the same. That is why I have an external LDG AT1000-ProII on standby.
« Last Edit: October 20, 2022, 1911 UTC by ~SIGINT~ »

Offline RobRich

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Re: Dipole antenna seperation
« Reply #5 on: October 20, 2022, 2149 UTC »
Another option is adding a single loading to the longer leg of your 80m OCFD to obtain either 30m or 12m.

https://hamwaves.com/cl-ocfd/en/index.html

I would opt for the 30m option if going that route, then deploy a simple ~9.4' 12m quarter-wave vertical over a few ground radials if wanting the upper WARC band.

Alternatively, for an off-the-shelf vertical, an Imax 2000 would probably get you 15m to 10m with rig's your internal tuner. Maybe even 17m, though YMMV.



All the above said, I probably would just redeploy your 80m OCFD as before, then a decent space from there add a separate 30m vertical over like 8 ground radials. Add another 8-24 ground radials as time and effort permit. Unless you are running low QRP, even a base-loaded ~11.5' (1/8w) vertical should suffice for 30m cw and digital work. Loading would be around 7uH, so like 8 or so turns in a short 4" diameter coil. A 13' fiberglass crappie pole is around $12 at Wally World.
« Last Edit: October 20, 2022, 2307 UTC by RobRich »
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Elf36

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Re: Dipole antenna seperation
« Reply #6 on: October 21, 2022, 1323 UTC »
Thanks for the info. I am actually planning on building a 1/4 Vertical for 30M. I have a perfect fiberglass pole that will give me the 24' height for 30. I really miss being able to operate on 30M and have always wanted to try a vertical. On a separate note regarding fan dipoles - I plan on building a separate antenna for using a SDR RX rig. It would be at low height, but I have about 150' of room to play with. Would a fan dipole with elements for say 3 Mhz, 6 Mhz, 9 Mhz and 13 Mhz (RX only)have an advantage over just making the dipole legs around 75' each (150') total? This may be a ridiculous question. 

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Re: Dipole antenna seperation
« Reply #7 on: October 21, 2022, 1725 UTC »
Until my recent relocation, my primary antenna had always been a T2FD, the Barker & Williamson, setup as an inverted v. That antenna served me very well for probably close to 20 years. Kind of best of all worlds. It was certainly a far more quieter antenna vise my current end-fed longwire. Now that the leaves are starting to fall off the trees I am working on anchor points to put this antenna back up. Unlike my previous location, I have the AC and utility (cable / phone) lines coming in pretty much at the centre of the house which I now have to contend with. It would have been a lot easier if the lines came it at a corner of the house.

When I was in Toronto I had the T2FD, the end-fed, the Wellbrook loop a vertical and a mini-whip for HF. The T2FD and the loop were my go to antennae. I'm thinking of installing a DX-Engineering coax switch outside to switch between all of my wire antennae and then only have 1 feedline coming in. Obviously the loop and mini-whip still need there own feed line because of the bias-t power requirement. When I removed all of the cables from my Toronto location I think I pulled out about 8 coax plus control cables for the rotator etc ... Not this time around. I'm going to try and keep it simple.

Offline RobRich

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Re: Dipole antenna seperation
« Reply #8 on: October 21, 2022, 1949 UTC »
Thought I remembered you buying a fiberglass pole for a 30m vertical. I would go that route and forget about trying to redesign the OCFD just to get 30m tuneable.

I agree with ~SIGINT~ about a T2FD. Great receiving antenna. The biggest downside is visibility assuming that is an issue.

How is the local RF noise situation? If it is not too bad, you might find a small active antenna to offer a good enough receive-only solution for your SDR, thus leaving your second spot open for experimenting with other wire antennas.

For example, it appears you probably have enough room for something like an 160m inverted-L if you can put down some ground radials or even a couple of loaded elevated radials. Alternatively, perhaps a loaded 160m dipole. Either would be a cloud warmer at such low heights, so mostly NVIS, but it would get you on the band.

Just some random thoughts.
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148' + 60' Loops-on-Ground | 31' Vertical | 18' End-Fed Vertical | PA0NHC MiniWhip

 

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