DX Engineering DXE-HR-2P Instruction Manual

Hot Rodz®

Adjustable Capacity Hat

for Screwdriver Antennas

DXE-HR-2P

DXE-HR-2P-INS Revision 1f

Shown installed on an
optional screwdriver antenna

© DX Engineering 2019

1200 Southeast Ave. - Tallmadge, OH 44278 USA

Phone: [phone] ∙ Tech Support and International: [phone]

Fax: [phone] ∙ E-mail: [e-mail]

Introduction

DXE-HR-2P - Parts List

Quantity

Description

1

Stainless Steel Mast, 22" with 3/8-24 threads on both ends

1

3/8-24 Trim Nut (installed on bottom of the 22" Mast)

1

3/8-24 Hex Nut (installed on top of the 22" Mast)

6

6" rods

6

12" rods

6

24" rods

1

Hot Rodz® Hub with internal threads

1

Allen Wrench

8

8-32 x 1/8" - Set Screws (includes 2 spares)

1

3/8-24 x 1/2" long Stainless Steel Hex Bolt

1

Rubber Washer

24

Black Tip Caps, for rods

The DX Engineering Hot Rodz® is an adjustable capacity hat system that increases the
efficiency and gain of screwdriver antennas. Hot Rodz® achieves this, in part by enabling the
use of a smaller coil, so that RF energy typically lost trough a larger coil is transferred to the
antenna for a more powerful signal. W reciprocal action occurs in the receiving mode so that
the received signal increases by a similar amount.

Part List for the DXE-HR-2P Hot Rodz®

Additional Requirements

You may need the following items that are not included in this package:

SWR Meter: The Frequency Selection guide will help you get close to

the ideal antenna size, but a SWR meter is necessary to
fine-tune the final assembly.

Antenna Matcher: To match the impedance between your feedline and

antenna, use a mobile matcher such as the DXE-MM-1
Dual Impedance Transformer

Guy Ropes: Depending on the configuration of the Hot Rodz®, your

antenna system may require additional support. SYN-
DBR-125-100 1/8 in. Diameter, Dbl-Braid
Dacron/Polyester Rope may be used.

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Theory of Operation

When an antenna is shortened from its natural resonant length, the feedpoint becomes capacitive. To offset
the additional capacitance, it is common to add inductance in the form of a loading coil to restore the
resonance. However, the additional inductor increases the resistance, which increases the loss of the antenna.
Hot Rodz solve this problem by using a capacity hat - a series of horizontal rods that counters the effects of
shortened vertical antennas.

A capacity hat can be thought of as an extension of the Marconi antenna design that adds horizontal elements
to the design of an otherwise vertical antenna. The currents in Hot Rodz® horizontal elements offset one
another and preserve the vertical polarization of the related wave.

Antenna Length and Performance

The more the antenna is shortened, the larger the coil necessary to compensate. However, the larger coil
increases the resistance and consumes more power, resulting in greater loss. The loss resistance is also seen
at the feedpoint and is partly responsible for making it easy to match this shortened antenna. to the 50 Ω
feedline and radio. The base impedance will drop to about 16-22 Ω after the Hot Rodz® system is added.

We suggest using a mobile matcher, such as the DXE-MM-1 - Dual Impedance Transformer, to ensure your
system operates at optimum efficiency.

The impedance at the feedpoint consists of the radiation resistance, coil loss and ground loss, with the

radiation resistance of a full size vertical antenna equal to about 35 Ω. As the antenna is shortened, the

radiation resistance decreases, but the other losses remain or even increase. To reduce the loss in a loading
coil, we try to use one with the least inductance and resistance that still resonates the antenna.

Greater Efficiency at Half the Size

If the base impedance is reduced from 50 Ω total to 20 Ω total when the actual radiation resistance of the
antenna is 3 Ω, then you will have more than doubled the RF power coming out of your antenna by raising

its efficiency from 6% to 15%.

Half the Length on the 10-80 meter Bands

The Tarheel model 200A screwdriver antenna uses a 6-foot whip for the 10-80 meter bands (3.1 - 28.5
MHz). When combined with a Hot Rodz® system with three 12-inch rods, a 22-inch mast and a safety
spring, the total of the antenna is 6-feet 11-inches, less than half the size of the standard 12-foot Tarheel. This
makes an enormous difference in how often you have to get out of your vehicle to shorten the antenna.

The screwdriver antenna covers large potions of the HF spectrum with just three 24-inch rods, but antenna
efficiency increases with a greater number of rods. Using more (or longer) rods is like extending the whip on
a conventional antenna. The reason is that Tarheel and other screwdriver antennas are examples of the art of
engineering for the mobile environment. Due to design considerations for screwdrivers, the Q (a measure of
efficiency) of the coil is low - approximately 75-100 - and the smaller the coil, the better.

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