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MARCONI 'T' ANTENNA CONFIGURATION

Adapting the main HF Doublet Antenna as a type of T Antenna was achieved by bonding the ladder line in the shack and connecting directly to the T-Match antenna tuner.


Principal of the 'T' Antenna

The Marconi T Antenna is essentially a shortened vertically polarized monopole antenna that is used for lower frequency bands, typically below 4 MHz and ideal for use on the 160m (1.8MHz) amateur band.

Vertical antennas are ideally suited to long distance broadcast type communication due to the low angle of radiation and Omni-directional characteristic.  At low frequencies a full quarter wavelength (λ/4) monopole antenna as shown in Fig 1 may be impractically large, therefor a simple solution is a T Antenna. The T section of the antenna will produce RF currents directed in opposite directions as shown in Fig 2, causing the radiated energy to be cancelled out far from the antenna. The two T arms act as a capacitance hat increasing RF current in the upper portion of the shorter vertical section, increasing the radiation resistance that will allow the antenna to be more easily matched to the feed line and improve the efficiency of the system.

Fig 1 RF current distribution on a λ/4 monopole antenna Fig 2 RF current distribution on a short monopole T antenna

  

The All Band Doublet configured as a 'T' Antenna

The main HF antenna is an All Band Doublet fed with ladder line and matched with a T-Match antenna tuner including a 1:4 current balun that gives access to all HF bands including the 160m band. With a view to get better performance particularly on lower frequency bands this antenna was reconfigured as a type of Marconi T Antenna by simple bonding the ladder line in the shack and connecting directly to the T-Match antenna tuner.

The T Antenna requires a counterpoise to work well and this will achieved at least initially by using the galvanized steel building roof to serve this purpose.  

 

Fig 3 Multi-band dipole antenna configured as a type of 'T" Antenna

 

Basic Multi-band dipole antenna configured as a type of 'T" Antenna

(1)  Inverted 'V' Dipole 42Mtr total length

(2)  450 Ohm Ladder Line 7.5Mtr length.

(3)  Junction box (Ladder line - twin heliax interface)

(4)  Twin heliax 3.5Mtr length.

(5)  Twin SO259 bulkhead sockets

(6)  T-Match Tuner. See T-Match Tuner

(7)  VSWR Meter.

(8)  HF Transceiver.  

 

Photo#1 Junction box (Ladder line - twin heliax interface)

Photo#2 Junction box (Ladder line - twin heliax interface) assembly

Photo#3 Twin SO259 bulkhead sockets

Photo#1 Junction box (Ladder line - twin heliax interface) Photo#2 Junction box (Ladder line - twin heliax interface) assembly Photo#3 Twin SO259 bulkhead sockets and Ground connection.

 

Antenna Modelling with MMANA-GAL

 

Fig 4 All Band Doublet modelled with MMANA-GAL in the normal configuration at 1.825MHz.

 

Fig 5 All Band Doublet re-configured to a 'T' Antenna and modelled with MMANA-GAL at 1.825MHz.

 

Fig 6  'T' Antenna 3D radiation pattern modelled with MMANA-GAL at 1.825MHz.

 

WSPR Measurements

Comparison of the Multi-band dipole antenna with the same antenna configured as a  'T" Antenna used for operations on the 160m amateur band.

WSPR data has been graphed using Excel. Horizontal axis = UTC Time, Vertical axis = dB below the noise floor.

Antenna#1: Multi-band dipole with T-Match tuner and 4-1 Current Balun

Antenna#2: 'T" Antenna configuration with T-Match tuner

VK6YSF WSPR Signal received by:VK2KRR

Distance: 2832km

Grid location: QF34mr   

dBm: 37

 

 

 

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Page last revised 25 November, 2023  
 

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