MORAD VHF-156HD Antenna and Mounts

[jimh]

I set out to write a short article in this forum about the MORAD VHF 156HD antenna and its mounting system. In the process of studying MORAD antennas and mounts, I found there was more to explain and describe than seemed appropriate for a forum article, so I wrote a REFERENCE article about the antenna and its mounts with a number of in-line illustrations and several hyperlinks to more information. See

The MORAD VHF-156HD Marine Band Antenna
and Mount Options
http://continuouswave.com/whaler/reference/MORAD.html

The executive summary: the MORAD VHF-156HD antenna is a commercial grade, rugged antenna with a number of mounting systems. It is an all-metal antenna and compared to consumer-grade lower-tier fiberglass antennas is significantly more expensive but also significantly more durable.

This thread can be used to make comments or ask questions about the antenna and the REFERENCE article.

[David Pendleton]

Nice work Jim, as usual.

I don't even remember how I became aware of the MORAD antenna. I was a little perplexed by the mounting "system" when I bought mine, but the folks at MORAD were more than helpful.

Regards,
Dave

[jimh]

In September 2020 I installed a MORAD VHF-156 HD antenna on my boat. I describe the installation process in a lengthy article at

A Captivating Saga of VHF Antennas
viewtopic.php?f=13&t=5734

[jimh]

My curiosity about the dimensions used in the MORAD VHF-156 finally prompted me to take some measurements. The upper rod or whip is clearly the radiating element and is 30.5 inches long. The lower encirling tube element is 22.5-inches long. This gives a total length of about 53-inches or about 4-feet 5-inches

A wavelength λ at 156.8 MHz in free space in inches is equal to the speed of light in inches (1.1803 x 1010) divided by the frequency in Hz (1.568 x 108)

 λ  = 1.1803 x 1010 / 1.568 x  108 = 75.27-inches

This means the two element lengths mentioned above in terms of wavelength at 156.8-MHz are

Radiating element = 30.5-inches/75.27  =  0.4052-wavelength
Tube element = 22.5-inches = 0.299-wavelength
Overall length = 53-inches = 0.7042-wavelength

A basic analysis suggests the upper rod or whip is a radiating element that is shorter than a half-wavelength and the lower tubular element is a matching network or counterpoise and is somewhat longer than a quarter-wavelength. The radiating rod likely extends all the way to the bottom of the encircling tube, and the rod and tube are connected together there. It is also likely that the transmission line outer conductor is connected at that junction. The inner conductor of the transmission line is likely extended upward inside the encircling tube and connects to the radiating element at a point selected to provide the best impedance match.

The use of a larger-than-normal diameter for the radiating element improves its bandwidth by lowering the Q. Similarly, the large diameter encircling tube also has reduced Q and improves bandwidth.