NON-resonant feedline dipole (NRFD)

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nanZor

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How about a slight rx-only spin on the classic "RFD", or "resonant feedline dipole"?

With parts left over from my loop-on-ground and dipole-on-ground projects, I had enough to make a slightly different antenna, that is purposely NON-resonant. So far, works great up in the air, and of course, on-ground too.

If you search for "resonant feedline dipole", you'll see that is is basically a quarter-wave of wire attached to the center conductor of a piece of coax (braid not connected), and a quarter wave of coax away, a common-mode-choke is applied. The idea here was to be "resonant" on a single band.

But I twisted that idea a bit to be purposely NON-RESONANT for rx-only use, by using a 9:1 transformer at the coax-to-wire feedpoint junction, and instead of the "feedpoint" being in the center, I offset it by about a 3rd. Like the classic RFD, I *wanted* common-mode interaction to be choked later down the line, so my transformer is sharing a common ground, ie an auto-transformer, one that is NOT ISOLATED.

My objective: 160 to 20m coverage. That means I'm not going to exceed about 30 feet *total* for the antenna itself (coax and wire). This is more important for my on-ground pattern, since exceeding a half-wave at the highest frequency will start to rapidly skew the more or less omni pattern.

Antenna: ( I used what I had laying around. 9-foot coax jumper and a NooElec 9:1 transformer, as set by oem as a shared ground). Here's the layout:

MFJ 915 ferrite choke > 9 feet of RG8X > 9:1 autotransformer > 21 feet of wire

The actual feedpoint isn't critical. Got a 12-foot coax jumper? Use an 18 foot wire. I think any coax jumper in the range of about 6 to 15 feet (and adjusting the length of the wire for a total of 30 feet) will do just fine. Experiment - use a 4:1 if you have that handy, but make sure it is an autotransformer, NOT isolated. Easier to come by anyway.

So far the results are about the same as my on-ground loop. In the air I have much more signal of course. I supported the lightweight Nooelec 9:1 transformer on the coax and wire with a popsicle stick for strength. I think I have additional common-mode choking since my feedline is on the ground. I choked it at the rig too with another ferrite choke, and noticed no major difference.

On ground, my SWR, if that really means anything to this rx-only application is no higher than 4:1, even at 160, where I still hear my squirrely little noisemakers in the neighborhood down low.

Note: I didn't go nuts over this thing. Use a better common-mode-choke like a MyAntennas CMC-0510-R. Perhaps a nicer 9:1 with better specs, or wind your own, but be sure NOT to isolate it. Lots of of transformers from the garden variety "end-fed" antennas that use a 9:1 might be in your drawer right now. :)

Thumbs up! Aside from hanging it in the air in a variety of configurations, it would also make a nice option for on-ground users that have a small side-yard, maybe just a long planter, and can't have a feedline coming off a regular loop or dipole on ground in the backyard.
 
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nanZor

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On ground users: This antenna is floating from ground, at least prior to the choke, so don't accidentally let any coax jumper shells short to ground. Tape / isolate them. I was puzzled at first when I first laid it down. Tape over the shells fixed it. :)
 

nanZor

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Don't tell Mike, but I pulled up my on ground loop for this. Not because the loop didn't work, it is just that this version has similar performance, and is more convenient to use at my location.

End-fed antenna users: easy to put one on ground. All you need is a GOOD rf-isolator / common-mode choke. The MFJ 915 will do, however I prefer at least 5K or more choking impedance, so the MyAntennas CMC was put in place.

Example: The classic "EARCHI" end-fed antenna with 30 feet of wire attached to a 9:1 autotransformer, sometimes comes with recommendations to run 16 feet of coax, with a common-mode choke at the rig.

Guess what? In reality that is a off-center-fed dipole with the feedpoint at about 30% of the way inwards. :) If you were to lay this down on the ground, you'd do fine, although above about 10 mhz the pattern will start to skew away from the omni / small-vertical-loop pattern. That is, the wire AND the coax prior to the choke should not exceed a half-wavelength for us on-ground users who want a nice pattern. So adjust your wire and coax jumper length accordingly if you want to put the feedpoint at a certain percentage of offset.

Note that most of my feedline is on ground itself, in fact much of it over cement with only a small rise to a low window into the shack. Anyone in an elevated position running the feedline high off the ground may need much more common-mode choking, or perhaps a true galvanically-isolated unit as well.
 

pjxii

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Where do you come up with all these antennas ideas? I think experimenting like this is brilliant, and I look forward to reading your posts in 2019.
 

nanZor

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None are really my ideas - KK5JY got me hooked that it was indeed possible which leads to research elsewhere.

This choked-ocfd might be useful for those employing them as phased arrays, where a feedline coming out of the ends might be more convenient physically.

My heart just goes out to kids aged from 6 to 60 (or even 80), that may be on the fence about giving up HF monitoring completely and help provide some actual tested solutions that are reasonable.
 

nanZor

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I'm also cheating by using antenna modeling and an antenna analyzer. :)

For example, I simulated a 50 foot feedline attached to this antenna by adding a 50 foot wire to the end, and placing a 5K resistive load where I wanted it. Without the 5K choke, it is a totally different antenna, approaching a very small unterminated "bog" at 14 mhz with screwy patterns.

I'm no antenna expert, but I model the antenna to see the trend, and put boots on the ground in the backyard with the analyzer to see if the trends are at least similar. If not, something is definitely wrong. It doesn't have to be exact, but the unexpected sends me back to the drawing board.

Just a reminder that modeling with EZnec (or others), and antenna analyzers are great tools for swl'ers, not just amateurs.
 

pjxii

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I don't know anything about computers and websites and such, but I think you ought to look into having your own webpage describing the details of all of these SWL antennas that you've built and tested, having the information in one place would make access easier than digging through this forum to find things.

I've collected a number of receiving antennas books over the years waiting for the day when I have a big yard to try a few of the more out-of-the-box designs. A random wire high and in the clear will do a great job but experimenting with different ideas is not only fun but can be even more effective in certain situations.
 

nanZor

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You have a good point, and that would give RR a needed break for sure.

Re the "out of box" designs -- most that I have run across disguise the fact that they are using the common-mode (outer covering of the coax braid) as the real antenna and have questionable or impractical to build qualities for little gain.

Here, we are blatantly using part of the common-mode, and then choking it off before reaching the feedline.

The only caution I'd say for those not experienced with s/n ratios, there is a fine line between a dummy-load, and something that actually works like it should with reduced gain.

Another tip - anyone trying these on-ground antennas may be doing so with less-sensitive, perhaps tired radios - or in some cases older amateur transceivers outside of the guaranteed ham bands, and something like an MFJ 1045 preselector/preamp would be a good investment. You don't need much amplification - just enough to make a difference when you remove / attach the antenna. No ear-bleeds please. :)
 

nanZor

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Off the shelf components tested and moisture warning:

Ok sure, the Par EF-SWL transformer with ground jumper attached works just fine in this configuration. Closely matches what EZnec said it would. Great build and hardy enough to use on ground as is.

BUT, what about other off the shelf stuff? Especially if one is also an amateur, and might accidentally pump some power into it, and destroy the rx-only transformer?

4:1 *UN-UN*s works just fine here too. Forget about baluns, be they voltage or current - you want an UNUN. Might be easier to obtain than a specialized 9:1.

Example: two 4:1 baluns which performed POORLY: an MFJ 913 current balun. A W2AU voltage balun. Do not use. Waste of time. Easily seen on analyzer and easily heard poorly at receiver above about 5 mhz.

An LDG 4:1 UN-UN, (not to be confused with their similar 4:1 balun that also has a green case sometimes) does just fine, and again - matches what EZnec says it should. As an "indoor" device, it needs protection, so the classic tupperware surround might do.

There are many un-un options, including balun-designs llc and others that are suitable if properly protected or weatherized already.

WARNING:
Don't let the coax shells themselves be shorted to ground. Insulate them with tape, or better yet something like coax-wrap or similar. Also, aside from simple rain, ground moisture itself can quickly degrade the jumper with moisture. So if you make this antenna somewhat permanent, then be SURE to insulate the shells.

At the end of the day, a relatively inexpensive off-shelf solution is to use an LDG 4:1 UNUN for the transformer, and an MFJ 915 ferrite choke at the end of the jumper.

Plenty of room to go nutso about the UN-un and choke with other brands. We won't go there. I'm satisfied with the LDG (tupperware protected) and the MFJ myself and saved the other stuff for my more critical "in air" antennas.
 
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nanZor

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ON-GROUND update:

I am totally satisfied with my current configuration on-ground as compared to the on-ground loop. I put the feedpoint in the center so as not to make the pattern lean at higher frequencies (18-21 mhz), and since the coax jumper being part of the antenna itself has it's own built in capacitive reactance, I thought *maybe* I'd try and balance it some with the wire part, by using zip-cord just connected together at each end. Who knows. Seems pretty handy *looking* like an end-fed, but it is really a center fed dipole.

30 feet of feedline to:

FERRITE rf choke >
16 foot rg8x jumper >
Ldg 4:1 UN-UN (ground terminal unused, purposely using common mode of jumper) >
16 feet of zip-cord paralelled together, to emulate the capacitive reactance of the coax. (Will it help maintain the pattern - who knows. Just experimenting with no way to prove pro or con as compared to a single wire)

Coax shells taped up so as not to short to ground.

I can't model things like phase shifts of the un-un, and what that might do to the pattern, but so far I'm pleased. Lot's of listening ahead!
 
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nanZor

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Choke notes regarding *on ground* use:

While I still *prefer* the CMC-150-R choke from myantennas, performance seems similar to the MFJ 915 although I haven't scoured the bands enough to find the corner case differences. BCB for sure the myantenna. Since I wanted coverage up to about 21 mhz or so, I left the MFJ 915 inline afterwards for good measure.

But the point is that with BOTH the antenna AND the feedline on the ground, in an rx-only environment, choking requirements may seem to be lessened to a degree as compared to a feedline that runs up in the air - ie use what you have, but be real about it. A single snap-on is useless.

For sure, removing the choke immediately makes the *entire* feedline part of the antenna as seen by EZnec modeling where it no longer follows the predicted swr measurements slope as seen by the AA-54 analyzer.

While not as seemingly simple as a true center fed dipole on ground, but for those that absolutely need the feedline coming off the end, this works.

UNUN notes - the 4:1 seems to do well with my relatively short feedline. If one is running a long feedline, then a 9:1 unun with it's lower swr, may help in cases where high frequency monitoring (14mhz +) and additional swr loss is a concern.
 

nanZor

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First DX *ever* heard on 160 meters! (a band so noisy for me I ignored it for decades - not any more!)

From LA to the Bahamas, copied a C6 station cw armchair strength near 1.834 mhz. Middle of the night though...

Makes sense - the main lobe of the on ground dipole is pointing that way for me. On 160 with only 30 feet of wire on ground, I was expecting to only hear the guys with low-mounted antennas with high angles. I've heard plenty of that, but this is exciting, proving that the antenna is not JUST an nvis antenna. And that the special configuration here seems to be working like it should so far...
 

KB4MSZ

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"but this is exciting, proving that the antenna is not JUST an nvis antenna"

Maybe there is hope for my full length but low height (12') horizontal dipole I installed at my office property a few weeks ago. Haven't had much operating time yet but hoping at least some energy is just going straight up.
 

nanZor

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Well, up at 12 feet we have the spherical pattern similar to the on-ground version. The lowest looking bidirectional lobes are still broadside to the wire. Yes, for transmitting purposes a lot of energy is being wasted. Not to mention being very narrowband and cable loss / swr loss is a big deal outside of resonance.

BUT, for an rx-only antenna, for a single band (like a 33 foot dipole for 20 meters) it can still be effective, although s-meter smashing strengths won't be there. You still have good directivity down low, albeit reduced in overall signal strength of course compared to one up much higher.

What might be surprising is dropping it all the way to the ground and listening again. If you like, put a 4:1 or 9:1 balun on it, and choke it off so as not to make your feedline part of the antenna especially if you are using an "autotransformer" style. The ground loss helps broadband the antenna by making the cable swr loss lessen. It's the classic game from yesteryear between "31 / 43 foot non-resonant vertical" where one juggles cable loss vs swr loss - that kind of thing.

When we do that on ground, of course signals are going to be anywhere from about -10dbi on 20 meters, -20dbi on 40m, and -30dbi on 80 meters etc.

BUT, being on ground is NOT the same as just punching in your rig's attenuator! There is a difference in s/n between the antenna up in the air, and one on the ground. A dx-er on a solitary island in the ocean might not notice a s/n difference, but for most of us urband city dwellers, it can be very noticeable. Wait for the bands to open before making any judgements.
 

KB4MSZ

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Oops, I meant to say I'm hoping at least some energy isn't going just straight up.

I installed 3 dipoles, none have baluns right now. I used 1" PVC T-fittings for the feed point, with the bottom T-intersection plugged and a SO-239 connector installed, see photos.

The 40 meter dipole covers the band easily
The 80 meter dipole has just over 200khz coverage at 2 SWR limits
The 160 meter dipole has 120khz coverage at 2 SWR limits
All three are resonate at their design frequency without reactance, which surprises me. All three are at 12' to 15' heightFeed Point 1.jpgFeed Point 2.jpg
 

nanZor

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KB4MSZ - kind of OT, but just mentioning when I say you are wasting power does NOT mean to give up!

There are PLENTY of people out there with low antennas, who are ALSO "wasting power" by not having lobes to work the big dx. However, if you want to just have fun communicating, I *know* there are a lot of people listening at short skip angles - seen often when the contesters show up!

So, don't be afraid to put out a CQ. More than once. Sometimes with no reply. With the advent of spectrum displays, you may well be seen as well as heard by more than you imagine - even with your low dipole.

P.S. - with my own 30 foot dipole (or 60 foot loop), I listen nightly with armchair copy to the guys in Port St. Lucie on 40m ssb for hours on end. From LA. Signals don't even budge the s-meter, but they are clean and comfortable to listen to. If I pumped some power into my on-ground antenna, it would only make the worms happy. :)
 

nanZor

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Nice pics!

Right - you know it ain't perfect, but you soldier on *knowing* that it isn't so. Don't let the lack of "perfection" stop you from communicating - which would be a total shame.

Heh, drop that 40 meter dipole to the ground, right on it - not a foot above but touching the soil (insulated wire naturally) and take a listen across the bands for an rx version.
 

KB4MSZ

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"Heh, drop that 40 meter dipole to the ground, right on it - not a foot above but touching the soil (insulated wire naturally) and take a listen across the bands for an rx version."

Unfortunately that isn't an option here at the office. The commercial landscaping crews would get whipped to death :)
 

nanZor

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Beefed up my game:

I like this on ground rfd so much that I beefed it up to withstand harsh on ground treatment from sprinklers, ground moisture, kids, and general mistreatment..

I'm using the 9:1 UNUN from Myantennas for the transformer, and the CMC-0510-R as the choke, along with a sprinkling of snap ons following it for forays well above 15 mhz....

While the LDG 4:1 indoor UNUN (the "RU-4:1" version) enclosed in tupperware was doing ok, this bad boy from Myantennas won't break if I step on it. :)

Analyzer responds accordingly, and I'm anxious to see if I can catch the rest of the 160m contest.
 
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