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I've recently learned about linear loaded dipoles as one way of building a physically shortened antenna. As I understand it, any shortened antenna will have reduced bandwidth and linear-loaded dipoles suffer from this as well.

I was thinking to counteract this by adding a second parallel element about a foot from the first, folded in the opposite direction created a linear-loaded cage dipole.

  • Is this a feasible solution? Put another way, would it actually give me usefully increased bandwidth?
  • From a terminology standpoint, does it require a third element to be a cage dipole?
  • Is there an all-around better option for an 80m antenna in ~70ft of linear space?
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  • $\begingroup$ (Apologies to everybody who's answered my questions so far. I'm almost out of weird antenna questions, I think.) $\endgroup$
    – William
    May 2, 2017 at 15:42
  • $\begingroup$ Read up on the Chu Harrington limit first to check that what you're trying to do doesn't violate that rule. There are also some design ideas on Wikipedia and elsewhere, essentially thicker is better. Linear loading might be less lossy than centre loading, but it doesn't change the available bandwidth much. $\endgroup$
    – tomnexus
    May 3, 2017 at 2:20
  • $\begingroup$ @tomnexus - I hadn't heard of that before. So it sounds like if my antenna is less than 25.4m (82ft) in diameter, the Chu limit applies. Thanks! $\endgroup$
    – William
    May 3, 2017 at 6:16
  • $\begingroup$ @tomnexus - I may be doing the math wrong, but I'm coming up with a minimum Q factor of 2.9 for an antenna that fits in a sphere of 10.6m in radius (70ft in diameter). At 80m, that would give me a bandwidth of 1.3Mhz - I don't think I'll even be approaching that! $\endgroup$
    – William
    May 3, 2017 at 17:24
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    $\begingroup$ My take on "linear loading" - it's just a shorted transmission line, a low-loss low-cost inductor. Useful for multi-band antennas because it changes over freq differently to a real inductor. You're free to put differently tuned elements in parallel, whether they're different lengths or differently loaded, and it helps with bandwidth, to a point. You might get as much out of just making the cage as wide as possible, say 30 ft at each end, end-loading it, and finally tuning it with an inductor. $\endgroup$
    – tomnexus
    May 3, 2017 at 20:05

2 Answers 2

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An 80 meter dipole shortened to a length of 70 feet or less will require some type of additional matching circuit even with linear loading. The linear loading will help to reduce some of the reactive component but the resistive part of the impedance will also require some additional adjustment. This is typically done with an ATU (Antenna Tuning Unit).

Given that there is likely to be an ATU in the antenna system, there is no point in trying to increase the bandwidth of the dipole by building it in a cage configuration. The ATU will take care of any SWR bandwidth limitation by shifting the match point of the antenna system.

To maximize the performance of shortened antennas, the efficiency of the antenna system is normally of the greatest concern. This usually involves design trade-offs in impedance matching techniques. One of the most efficient methods for a shortened dipole is to use capacity hats at the end of the dipole. While this is not usable in all situations, a 40" diameter hat on the each end of your shortened dipole would go a long ways to obtaining an efficient match. Also consider the placement of the ATU and the losses in the feedline(s) due to SWR as part of your efficiency calculation.

The Chu limit is not applicable to your situation as this is typically used to be certain that the antenna can radiate the entire bandwidth of the modulated signal. Since our HF modulation methods are generally 12 KHz bandwidth or less, the calculation is meaningless. Even a very high Q antenna such as a small loop can fully handle the modulation bandwidth of HF signals. If you were considering HDTV type transmissions on UHF with a small form factor antenna, as an example, then the Chu limit might come into scope.

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  • $\begingroup$ I'm not sure I understand where the capacity hat would go in my case. A linear-loaded dipole basically folds back on itself and my gut says a cap hat halfway down isn't right. $\endgroup$
    – William
    May 4, 2017 at 18:15
  • $\begingroup$ The cap hat is in place of linear loading. It would typically be one or the other. $\endgroup$
    – Glenn W9IQ
    May 4, 2017 at 19:41
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A simple solution is to allow the parts of the full-sized dipole beyond your 70-ft limit to hang down, toward the ground. I believe this will have less deleterious impact on the dipole's driving-point impedance and radiation behaviors than will folding it. Since we're discussing compromises, it's worth noting that it's not necessary for the excess to take any particular angle with respect to the horizontal portion, but symmetry will help to preserve the dipole's natural broadside radiation pattern. Interaction with ground may affect the resonant frequency, so start long and trim to length.

Something similar could be accomplished using an "inverted V" configuration and turning the "excess" ends of the wire back toward the center support.

Remember that the ends of the antenna will have very high voltage, so keep them away from living and flammable objects.

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