The most UNDERESTIMATED antenna out there is the ____________________________________

[KC2SIZ]

Fill in the blank and please elaborate a bit.

[WB2WIK]

I think it's the multi-loop, stacked, 64-rhombic array.

Hardly hear anyone using one, but when you do, they're really strong.

[KA4EET]

The rotatable dipole.
Easy to make from old parts, light to turn, low wind load, and although no gain, it does have 50/50 directionality.

[NN3W]

The delta or diamond loop.

[K8JD]

coax fed dipole.
Most NEW hams want to work all bands with one random untuned antenna with expensive ladderline and tuners..
Pick an interesting band to try and put up a single band, coax fed dipole. Needs no tuner, can be homebrewed with less than $ 40 for wire/coax/insulators etc. Save your money for future Yagis and tower.
Want another band ?, Add two more pieces of wire and make a fan dipole.

Fill in the blank and please elaborate a bit.

[2E0OZI]

Coax fed monoband dipole - mine cost less than £2 to make and worked fine on 20m for chatting around Europe. Its odd that people would actually buy them.

[K7JBQ]

Halfsquare.

Only decent DX antenna I know that's afraid of heights.

73,
Bill

[W8GP]

POD (plain old dipole)

[KL7AJ]

The top loaded short vertical, followed closely by the DDRR, when properly installed. Problem is, nobody does the DDRR right. I had a fabulous DDRR on 160 for years....it worked better than any other equal height antenna when the ground is as lousy as ours!

Eric

[N3OX]

I have another vote for "the rotatable dipole" and other single-element rotatable cousins like full wave loops.

In a lot of situations, once you have the required support and rotator you might as well put up a beam, but I see many comments from people who seem to think they won't see any difference over their long, nully doublet or random wire until they have the GAIN of a beam, even if simply completely avoiding nulls would be worth 10dB or 20dB in certain directions.

I have used many single-element rotatable antennas and I've never been disappointed. At a former QTH I had really good luck with a rectangular 17m loop hanging from a tree like this:

http://n3ox.net/files/17loop.jpg

At this QTH my first antenna for 20m and 17m was a thin wire rotatable delta loop supported on fiberglass fishing poles:

http://n3ox.net/files/2017delta_lg.jpg

Even if you put up CROSSED switched dipoles for a given band, a rotatable dipole or other similar antenna will be around 3dB-4dB better in gain at the four 45 degree offset headings. If you can put a beam up, all the better, you probably should. But if you can't, a bidirectional antenna you can actually point on the right heading has a pretty substantial advantage.

The tree-supported 17m rotatable rectangular loop was turned by a Radio Shack TV rotor... It was really quite good and with the terrain and soil conditions I had at the old QTH I found I could work stuff on it that I couldn't even hear on my homemade 17m ground plane (which happened to be a big improvement over my long random wire). You probably don't want to put up a tower or strong mast with a real rotor and then waste it on a single-band bidirectional rotatable antenna but sometimes circumstances mean you can get "rotatable" more easily and cheaply than "unidrectional beam" and that, I think, is under-rated.