Which is better in my situation: Balanced dipole, or as much wire as possible in the air?

Question

I'm finally getting to the point of hopefully putting up a HF antenna. The idea is to start with a simple, straight wire dipole using trees as support points. (No traps or coils.)

However, with the layout of my property and the location of the trees thereon, putting up a fully balanced antenna might be difficult.

I'm not aiming for an antenna that is resonant on any particular frequency to begin with, so will be using an ATU in any case. I'm also going to be operating on most of the HF bands, so the feedline will essentially be random length at least on some relevant frequencies.

I'm aiming more for local-ish contacts than DX; with what I'm planning to do right now I won't really be able to get the antenna up high enough to get the takeoff angle down far enough for DX anyway.

The general idea for station setup is to have the transceiver, then 50 ohm coax to the ATU (which in turn is just inside the window, and which has a built-in 4:1 balun), then attached to its binding posts balanced ladder line (through the window) which eventually splits into the two legs of the antenna to actually radiate the signal. There will also be a bleeder resistor somewhere along the length of feedline, to help reduce the buildup of static, and a physical/electrical disconnection point between the bleeder resistor and where the feedline enters the house (allowing for true electrical disconnection of the antenna when I'm not on the radio). I'm aware of the need to keep balanced ladder line away from other conductors that it could parasitically couple to (as e.g. Hamsterdave illustrated just recently, though perhaps not quite as extreme).

With such a setup, which approach is likely to lead to better operating results, as well as less problems with RFI and friends?

• As much antenna wire in the air as possible, disregarding the relative feedpoint, possibly leading to an off-center-fed antenna?
• Keeping the antenna as balanced as possible, even if this means less wire in the air?

Bonus if answers also touch on how the choice is likely to impact antenna tuning (it'd be nice if I don't have to retune every 10 kHz).

The ambient HF noise level, as far as I have been able to determine by completely unscientific testing, is quite low, and I'd love to be able to make the most of that.

Answer 1

I think in practice you'll find it doesn't make much difference as long as there's a common mode choke or "current balun¹" at the feedpoint. In fact, if you get it off-center in just the right place you can get an impedance around 200 ohms on several bands. Even though you'll be using an ATU, there's a limit to how much mismatch they can tune out, and feedline losses go down with decreasing SWR.

When the feedline isn't in the middle of the dipole, the impedance between the common mode of the feedline and the two legs of the dipole are unequal, so you end up with common-mode currents on the feedline. This isn't much different from the situation with a coax feedline. Just use a choke.

The balun in the ATU does take care of interfacing with the ladder line, but it doesn't do anything about the feed arrangement: you still need a choke up there if you want to prevent common-mode current on the feedline, thus effectively making the feedline part of the antenna. At 100W or less you'll probably not get any RF burns, but it does mean your antenna is within a few feet of noisy electronics like computer monitors and switching power supplies in your shack. Not the best for receive performance.

A couple more points of note:

If the intention in disconnecting your antenna is to protect from lightning damage, you'll need more than a little gap in a switch to do it. See W8JI on grounding systems (The "Isolating or Disconnecting feed lines" section in particular) and How can I protect equipment against a lightning strike?

Your antenna setup is not going to have any effect on RFI with your neighbors except to the extent that you radiate less energy (not usually desirable in an antenna), put it farther from the neighbors, or direct the radiation pattern away from them. Feeding the dipole in the center or not will make no difference.

Once you have a 1/2 wavelength of wire in the air, more isn't really a benefit.

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¹ or is it a balbal in this case? Or a unbal? Amateur terminology is so confusing.

Answer 2

A fan dipole (element cut for each band and connected together at the feed point) is a bit more fuss, but a really trouble-free antenna. It will be resonant on each band. It is center-fed and horizontal, so will pick up somewhat less noise. Because the impedance is close to 50 Ω, a choke at the feed point will be more effective than on a non-resonant antenna with a different impedance. A good choke will be designed to present a 50 Ω impedance for differential mode currents and a high impedance (often over 5000 Ω) for common mode currents. That works best when the feed point is near 50 Ω, of course.

On my fan dipole, a 1:1 choke from Balun Designs dropped the noise floor on 20 m by about 6 dB.

With a tuner, you don't absolutely have to trim each element for resonance, but it might be worth the trouble.

Answer 3

I haven't been in your exact situation, but I have friends who have, and I would suggest that the answer is it depends. There are many variables involved. One important variable is the quality of your balun. Another is the kind of contacts that you'd like to make: local, DX, or both?

There's certainly nothing wrong with asking here for suggestions, but a big part of the traditional amateur radio experience is experimenting with antennas to see what works best for you. I suggest that you put up a dipole (it would actually be a doublet if you don't plan on using it only near its resonant frequency), or an unbalanced doublet, in such a way that you can easily add or remove wire later.

Upon reflection I'll venture an opinion: I would say that the antenna being slightly unbalanced, with the feed point somewhere in the middle third of the antenna, would be unlikely to cause you problems. Many people run antennas more unbalanced than that, but then the balun becomes more critical, so I wouldn't recommend a more unbalanced antenna for starters.

I also suggest that you pay attention to mechanical details. Most importantly, try to keep the ladder line as far away from metal as possible. You'll want to put up your antenna so that you can easily bring it down temporarily and put it up again; perhaps have the ends of the antenna attached to halyards, i.e. ropes attached to pulleys. Some sort of strain relief so that the wire doesn't get snapped in a storm would be a good idea.

I think you'll enjoy the process of experimentation much more than just accepting someone else's idea of what antenna would be best. Have fun!

Answer 4

I would prefer balanced dipole. That would solve some issues by itself.

With unbalanced antenna you would have to use chokes to fix symptoms like RFI, but does not fix unbalanced behavior of antenna so it is better to avoid need to use them.

Check Why aren't there baluns that do BOTH balanced/unbalanced transformation and common mode current attenuation? for some explanation about chokes.

If you plan is to use single antenna and ATU, then I guess using as much wire as possible is more viable option (I still prefer a balanced setup, or a closed loop). You will have huge losses anyway so more wire will at least compensate, and radiate more efficiently energy that your radio puts into it.

As already suggested, it is possible you will not find a solution in your first strike. It needs experimentation.

I would suggest reconsidering the idea of using a single antenna and think of some kind of multiband antenna or a combination, a dipole for the lower bands and a vertical for the higher bands.