If you omit it, the ground or the outer of the feeder provides it. It's theory versus practice! The aerial may be 'built' as a monopole, but the RF needs something to act against. I didn't intend to turn a sensible question into a debate on antenna theory, but as with most theories, there are differences of opinion. If a balun is located at the feed point, it will inhibiit any standing wave on the cable, leaving standing wave only on the single element. The current distribution in the single radiating element resembles that of a diapole, but with only one element hence 'monopole or unipole.'ДЄdmittedley, when fed with coax, the current in the antenna feed is going to be equal to that in the outer brade, but that is not a measure of 'balance' which should also take into account phasing. The advantage of the colinear over a quarter wave is that it does not need an earth or ground plane to make it efficient. If its fed 'up the centre' then it probably is a diapole! It is 'unbalanced' and has no second pole, hence it is not a diapole. fed at the end via a quarter wave matching stub and a transformer. The simplest colinear is an endfed halfwave. 'The Monopole or Unipole is an unbalanced radiator, fed against an earth plane a common example is the groundplane vertical' Tech details available if wanted - let me know the frequencies you're particularly interested in, and I'll give you dimensions to centre on those.ДЇrom a Radio Communication Handbook and the ARRL Antenna Handbook:

That'll be very much better than anything on your desktop, and certainly better than a broadband discone (or anything else broadband). Run the coax through any convenient channel/path to where you want the receiver, then (only then) solder on the BNC plug, and you're laffing. If you're half way decent with a soldering iron, build yourself a copper pipe dipole, use pvc pipe to support and separate the middle, solder on your coaxial cable (traditionally the inner goes to the element pointing upwards) and hang the whole thing from the rafter. I never wrote the rules for radio horizon. Pianorak Square roots are your friend! And it ain't my fault, guv, honest. I can span the globe with those on HF, so they ain't bad. There are six dipoles on one feeder (spaced 15 degrees or so apart), and one on the other. In the case of the transmitting dipoles in my attic, they're fixed to insulators on the rafters and there's no metal above the floor level apart from the feeder (and the nails in the joists, but they are a tiny fraction of a wavelength).

Interaction of a vertical dipole with the mast/pole isn't a problem as long as it's a reasonable distance away and/or not resonant. Outdoors is better than in the attic by a couple of dB when the antenna is new, and worse by more if damp and corrosion get to it. You can get similar results with a folded dipole half wave and a ground plane system - that will also have gain on low-angle signals. It will provide gain compared with a dipole, but at the expense of radiation angle - you won't hear high aircraft as well, if at all. The advantage of a colinear is that it can be other than a half wavelength - but construction is complex, so you have to buy commercially or be very good at precision building and testing/calibration. Available from Maplin for aboutДЇishhead A basic colinear is a form of vertical dipole (look at it!). Often a simple way to improve an airband receiver is to swap the inefficient rubber duck for a quarter wave length telescopic antenna. If you buy a good antenna, buy good quality cable to connect it! There is no point loosing all the antenna gain in the cable. The folllowing companies have a good selection of dedicated airband antennas. Colinears have more gain at right angles to the antenna but the gain is reduced off the end of the antenna (vertically above it) Outdoor antennas are the only answer to good coverage, vertical diapoles are not recommended as they interact with vertical poles and supports, a groundplane, or colinear design is much better.

'Desctop' implies indoor with very limited range! There is nothing magic about antennas 'Skyscan' implies a broadband scanner antenna: a jack of all trades but master of none! If you only intend to use airband don't buy a multi-frequency antenna.

Scanmaster antenna