Seeking advice regarding simple OCF dipole for SWL

[cornman]

Hey ya'll just wanted some advice regarding my antenna setup. Everything works quite nice im just looking to make things even better. Here is a little doodle that shows my antenna situation.

So I have my airspy hf+ discovery inside and the feed line comes in under the window and is connected to the sdr. The RG-6 feedline runs underground across the backyard (~100ft or so) then comes up a small tree. The feed point isn't that high off the ground, maybe like 12-16 feet snaked up the small tree.

The coax is split open and the shield and center are connected to separate antenna legs.

The leg connected to the shield runs up to a branch on a larger tree. Past that point, it lays flat across several branches. It zig zags slightly but is mostly under the center fed leg.

The center fed antenna leg slopes upwards at a roughly 45 degree angle and rests on a tree limb far up in the air.

This antenna is for shortwave receiving only and the frequency bands that im interested in are from 80m to 20m. Anyways, the coax shield leg was my first antenna attempt and is just a wire of random length (with it originally connected to coax center with the shield grounded). After my first attempt i wanted to try a multi band end fed antenna so thats what the roughly 66ft antenna leg was originally there for (trying to cut for 40m). I did a initial test run of it but found no difference in performance compared to just having the one random wire. Also, from what iv read it seems that, for a multiband end fed, the individual wires need to be of a specific distance from each other other wise it just acts like a larger random wire.

After some searching around i settled on the OCF dipole setup. It seems like it didn't improve signal strength but it did reduce noise which makes me happy.

So anyways again, just looking for ideas on how to improve the setup some more. Would a RF choke clean up the noise? what about one of those balun/unun things, would that make any difference? Am i right about the multi band end feds, that each wire needs to be of specific distance from each other?

 

[popnokick]

Yes, you need a 4:1 balun between the two “legs” of the antenna. And the two legs should ideally be 180 degrees apart (straight in line with each other) but at least 110 degrees or more apart if you can’t hang them in a straight line. In your diagram it appears the two legs of the antenna are very close to each other…. maybe even in parallel. Not good.

 

[prcguy]

I would ditch the OCFD idea since the legs are not running 180deg apart and you would need a particular type of 4:1 balun which is hard to find anyway. Instead stick a 9:1 balun at the feedpoint and make it an end fed then use a good choke balun in the feedline at the base of the tree.

 

[cornman]

I would ditch the OCFD idea since the legs are not running 180deg apart and you would need a particular type of 4:1 balun which is hard to find anyway. Instead stick a 9:1 balun at the feedpoint and make it an end fed then use a good choke balun in the feedline at the base of the tree.

If i was to make it a end fed should i attach just one leg or both? If i were to attach just one, which one should i choose? does it matter? would picking the higher up antenna yield better reception?

 

[WA8ZTZ]

The 9:1 balun (unun)connected to an end fed long random wire is IMHO the best approach if you want a quick and simple antenna that will work fine for your receive only application. If you do a search, all kinds of information available on how to build one. Otherwise, just buy a PAR EF-SWL and be done with it (you will also need an adapter to connect the RG6 type F to the SO-239 on the PAR matchbox). You will have to experiment as to whether or not to ground it or add a counterpoise to see what works best for you. Don't forget lightning protection.

 

[popnokick]

If you add a balun as the feed point for your 60 foot wire what you have will be nearly identical to this SWL Antenna sold by MyAntennas -
SWL antenna
It's a 60 foot long end-fed wire. You'll notice in the product description that it uses a 16:1 balun (rather than a 9:1). You could replace what you have with their SWL Antenna (includes the 16:1 balun).

 

[dlwtrunked]

The 9:1 balun (unun)connected to an end fed long random wire is IMHO the best approach if you want a quick and simple antenna that will work fine for your receive only application. If you do a search, all kinds of information available on how to build one. Otherwise, just buy a PAR EF-SWL and be done with it (you will also need an adapter to connect the RG6 type F to the SO-239 on the PAR matchbox). You will have to experiment as to whether or not to ground it or add a counterpoise to see what works best for you. Don't forget lightning protection.

I would do that also from probably the roof edge to high in the tree. I also encourage trying a inexpensive Youloop on a wood frame with the Discovery HF+ as it works shockingly well with the Discovery HF+ as the Discovery HF+ has high gain. Note I have seen two versions of the Youloop. One works better better than the other and is the one sold by AirSpy.

 

[prcguy]

Having the antenna feedpoint 100ft away in a tree and the wire heading away from the house can be very beneficial if you have RF noise making devices in the house and most do. You would need a good common mode choke in the feedline to make the best use of that setup.

I would also go for the shorter 65ft run that goes up high in a tree over the longer run that's closer to the ground. You may have a bit more noise from distant storms with the higher wire but you will pick up ground wave signals better and further out than the lower wire. 65ft is a good amount of wire and more can be better but if its close to the ground it kind of counters the benefits.

I was on a camping trip last week and brought my W6LVP experimenters loop kit, a Chameleon MPAS with 60ft of wire and a 40-10m EFHW with 64:1 balun and 64ft of wire and did some A/B testing with an Icom IC-705. I stayed in a cabin built into the side of a cliff in the Panamint mountains of a So Cal desert at the bottom of a valley with very steep walls and could only hear NVIS signals on the 40 and 80m amateur bands plus a handful of SW stations and some local AM BCB.

The W6LVP loop performed very well allowing me to null out competing stations on the AM BCB but the two wire antennas of similar length did give higher signal levels and a higher SNR. There was not much difference in AM BCB or SW reception between the 40m EFHW and the Chameleon MPAS but making a few contacts on 40m showed the 40m EFHW to be about 3dB better than the MPAS on transmit. The Chameleon MPAS would load up on 80m where the 40m EFHW would not so 40m was the only transmit test I could do since all other amateur bands were completely shielded by mountains and very dead.

So with all that I think 65ft of wire going up at a 45deg angle and a 9:1 balun with no counterpoise would be the best option for the OP and better than an active loop. Just make sure you put a good choke balun inline.

Everybody likes pictures, so here is a picture of the remote cabin I used for the antenna testing.

 

[cornman]

hey ya'll thanks for the advice, im 99% sure im just going to follow ya'lls advice make it an end fed with the 9:1 unun and choke.

regarding the length of the wire, im thinking about taking down both and soldering them together then putting the longer combined wire up as high as i can. Would a longer wire yield even better results? Also, can there be "too high" when it comes to these end feds? I read somewhere that it ideally should be like 1/2 the lowest desired wave length up in the air or somthing like that (for optimal performance) im very new to anntenna design and theory so please forgive my ignorance.

also just as a fun thought experiment, what if i took one of the legs down and set it up back up such that it has a horizontal V shape. I don't have enough space for a proper 180 dipole in a straight line but i could easily do this horizontal V antenna arrangement.

 

[popnokick]

For a receive-only antenna and general SW / MW listening (500 kHz - 30 mHz) you should get it up at least 30 feet or higher, preferably on both ends. But if not possible then a sloper with the high end at 30 ft (or better) is fine. Will it work at a lower height? Sure... but with fewer signals, and perhaps a lower noise level.