If your “two cables joined in the middle, held aloft on poles with a single feed down the centre” is a centre-fed wire dipole / inverted-V style tuned to 1420 MHz, its effective capture area is tiny compared with a 3 m parabolic or a 20 dB horn.
Numerically:

• 3 m parabolic (optimally illuminated, η≈0.7): gain ≈ 31.4 dBi, effective aperture Ae ≈ 4.95 m².
• 20 dB horn: gain = 20 dB (linear = 100), Ae ≈ 0.355 m².
• Half-wave dipole (≈2.15 dBi): Ae ≈ 0.0058 m².

So a single dipole captures ~850× less power than the 3 m dish (4.95 / 0.0058 ≈ 850). To match the 3 m dish’s energy you would need the equivalent of roughly 850 half-wave dipoles (or an aperture of similar physical area ≈ 5 m² implemented as a proper aperture or array).

---

How those numbers were obtained (brief)

Key formulas used:

• wavelength λ=c/f\lambda = c/f (at 1420.405 MHz → λ≈0.2112 m \lambda \approx 0.2112\ \mathrm{m});
• effective aperture Ae=λ2G4πA_e = \dfrac{\lambda^2 G}{4\pi} where GG is linear gain;
• dish gain approximation G≈η(πD/λ)2G \approx \eta\big(\pi D/\lambda\big)^2.

I used η≈0.7 for a well-illuminated dish with a good edge taper, a 3.0 m diameter, horn gain = 20 dB, and dipole gain ≈2.15 dBi.

(If you want the step-by-step math I used, I can paste it.)

---

Interpretation & practical points

1. A single two-wire centre-fed dipole is not a ‘collector’ like a dish.
   • Wire antennas couple to the E-field locally; their effective aperture is a function of gain, not their physical length/area in the same way a reflector is.
   • At 1420 MHz a simple dipole has a very small Ae (~0.006 m²), so it intercepts only a tiny fraction of the power a 3 m dish would.
2. Comparison to a 20 dB horn:
   • The horn’s Ae (~0.355 m²) is >> a dipole’s but still ≈14× smaller than the 3 m dish (4.95 / 0.355 ≈ 14).
   • So performance ranking (collected energy at 1420 MHz) roughly: 3 m dish >> 20 dB horn >> single dipole.
3. How to “scale up” a two-cable idea to match a 3 m dish:
   • You’d need either a true aperture (a conductive mesh/reflector ~3 m across), or a phased array of many elements with a total physical aperture ≈5 m² and appropriate phasing to get the dish’s ~31 dBi gain.
   • Practically that means hundreds of dipole elements (the ~850 figure above) arranged and phased — not achievable by a single two-wire suspended between two poles.
4. If your goal is simplicity / low cost:
   • If you only need to detect strong signals or do casual listening, a single dipole/inverted-V is fine and very cheap.
   • If you need the sensitivity of the 3 m dish (e.g., weak hydrogen line work, interferometry baseline), the two-cable centre-fed approach will not suffice unless you turn it into a large aperture or a proper phased array.

---

Concrete recommendations

• If you want dish-like sensitivity but prefer wires: build a wire-based aperture (a large rectangular mesh or many parallel wires spanning ~5 m²) placed in front of a small reflector or used as a phased array. That gives you a true aperture with Ae ≈ 5 m².
• If you’re after directional gain but want simple construction, consider a Yagi array or a small end-fire array of several elements — those give modest gains (6–15 dBi) with small footprints but still fall far short of a 3 m dish’s ≈31 dBi unless you make a very large array.

---