National RFís reference antennas are precision dipole antennas that offer a fixed 2.15 dB of gain over an isotropic antenna along the major lobe axis at specified frequencies (2.15 dBi). They are intended to be used for antenna field pattern and gain measurements, or general RF sensing in the laboratory. Mechanically, the units consist of a center hub that holds a standard female BNC connector, and connects to two machined brass element holders. The holders allow attachment of the various stainless steel elements, securing them in place with a set-screw arrangement. The assembly may be fed with either standard 50 ohm or 75 ohm coaxial cable. See the SWR discussion below for use with either impedances.
The Type NRD Reference Dipole assemblies can be ordered to any customer specified frequency in the 135 to 500 MHz range. Each order will be individually manufactured to the customerís frequency and tested for resonance and SWR at resonance.
Sold separately is a dipole holder assembly that engages the hub unit and allows mounting the antenna to a ½ inch plastic rod. With this holder, the antenna may be adjusted for either vertical or horizontal polarization, or any angle in between.
SWR and Gain Discussion
As stated, the reference dipole exhibits a theoretical gain of 2.15 dB at resonance over an isotropic antenna along the major beam axis (a power gain of 1.64). Also, the theoretical feed impedance of a dipole resonated to the operating frequency is 73 Ohms. Thus, when the reference antennas are fed with 72 or 75 ohm impedance coax, the SWR at resonance will be nearly 1:1, and the 2.15 dBi gain number may be used. When fed with 50 ohm line (which is conventionally accepted in the industry) the theoretical SWR will be the ratio of the load impedance to the feedline impedance, 73/50, or about 1.4 to 1. This equates to a reflection loss of only about 3.5 percent, which translates to nearly a 2 dB gain number (1.58 power gain) over the isotropic antenna.
The basic Type NRD antenna is sold with one center hub assembly and one set of customer specified frequency elements. Each unit will be prepared and tested to the customerís frequency. On the order page, please specify the frequency of operation down to the nearest 10 KHz - e.g. 137.62 MHz.
Additional element sets may be ordered to specified frequencies as desired. Please contact National RF with your specific requirements.
|Reference Antenna & Accessories||Price||Shipping|
|Type NRD Reference
(including one set of customer specified frequency elements)
specified frequency element sets
(Please call for discounted prices on orders of 2 or more specific frequency element sets)
|Antenna Hub mounting device and 3 foot section of mounting rod||$19.95||$7.50|
prices are for mainland United States
CA residents must add 7 3/4% sales tax
Overseas Customers: Please note that an additional overseas shipping charge will be applied to your order. National RF will notify you of the exact amount of shipping to your country via e-mail, after your e-order is placed. The customer is responsible for all and any customs or tarrif charges applied to your order.
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