Feed System Configuration — VDA Tuner Setup | Greyline
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Technical Note No. 2.2.1
Feed System Configuration: Tuner, Choke, and Coax Setup
Physics of the feed point
A Greyline 6061-T6 VDA is an electrical half-wavelength vertical dipole. The antenna operates without a radial field because both halves of the dipole are elevated — the feed point floats electrically above ground. Correctly configured, the feed system preserves that isolation and allows the tuner to present a matched load across the full operating range.
The signal chain:
Radio → ATU → RF Choke → Coax → Antenna
Radio → Coax → RF Choke → ATU → Antenna
Configuring the Tuner and Choke
A common assumption is that all antenna systems require a dedicated ground connection at the tuner. For the Greyline VDA, that assumption is wrong. The flagpole is a balanced antenna — both the upper and lower radiator sections are electrically isolated from earth. The off-center (OCF) feed point requires a balanced feed to work correctly.
A shack desktop tuner is the preferred configuration for roughly half of Greyline buyers — including RF engineers.
Two accepted configurations:
1. Radio → ATU → RF Choke → Coax → Antenna
2. Radio → Coax → RF Choke → Remote ATU → Antenna
The floating balanced output
Most modern tuners have an unbalanced output — the coax shield is grounded internally. To present a floating balanced output to the dipole, place a 1:1 current choke in the coax run at the antenna feed point. This suppresses common-mode current on the coax shield and keeps RF off the feedline outer conductor.
Build sequence — antenna through feed system
- Ground section. Thicker-wall 6061-T6 lower section sets in the PVC sleeve. No electrical connection to earth required.
- Feed point connection. Upper radiator (longer section) connects to the center coax conductor. Lower radiator connects to coax shield at the choke.
- Choke placement. Mount the 1:1 current choke at the antenna feed point, at least 6 inches above grade. A PVC standoff or wooden stake both work.
Ready to install the kit?
This page is the system theory. For the order of operations — ladder line, spacers, the interface block, and the one mistake first-time installers make — read The Greyline Feedline Kit: Ladder Line, Coax, and the Interface Block.
Agency and municipal deployments
Configuring a system for a Sheriff's office, Fire EOC, or emergency management facility? Greyline provides full technical documentation and architectural briefs for municipal and agency installations.
Agency Solutions · Capabilities Statement · Embassy Standard
Technical references
- VDA physics — vertical dipole fundamentals
- RF balance — isolation vs. symmetry
- VDA vs. 43-foot vertical — radiation plot comparison
- Commercial-grade installation standards
- Technical line: 435-200-4902
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