Update on portable HF vertical antenna (40-10m)

Couple of days ago I had posted a writeup on my trial AHVD antenna. While it seems very clean design, to get efficiency out of such asymmetric dipole requires careful adjustment of vertical element vs radials (asymmetric ratio). Elevated legs gives it further boost due to decoupling effects from lossy ground. It would have been nice to have properly marked telescopic Aluminum tubing to go for short trips. However, unavailability of such fine tubes here, drove me to assemble wires and mash them with Aluminum L angles, center boards, wires, nuts n bolts etc. I thought this is bit too much for my operations. What else I can do if have to cut down on poles required, remain stealthy and light weight?. 

Circling back to the basics, Why not ground mounted vertical with few(8 to 12) short ground radials?. I may lose bit of efficiency, but hey, anyways, I am not going to place this antenna on a poor ground for serious contests or DX operation. So, quickly turned the AHVD to simple vertical system with same Prolite tripod stand and single 5m Caperlan fishing rod.  Tripod was just at 4ft level where the central connector went in. I placed air core coil which was designed for 40m AHVD through the base of Caperlan rod. I removed base plastic cap of fishing rod and pushed it over the top of Tripod mast (with some rubber pad inside for cushioning) for around 6-8inches. I used around 4.5meters of wire to go vertical and 6 random length (4 to 5 meter long) of wires on the ground. Beauty of the system is that, I can quickly adjust the length of vertical wire which is held together to rod by few velcro straps and insulation tape. Also, Coil is not used for any bands above 20m. Anytime, I can convert it to elevated radial resonant system with right lengths as well. 

When I first checked SWR with my analyzer, wire was fully extended to the top of rod and resonance was at around 6.9Mhz with coil in circuit. I then reduced wire length ~0.5 meter lower and adjusted the coil winding to get resonance at around 7.1 Mhz 1.05:1 ish and almost flat from 7.01 to 7.2Mhz. 

Then I bypassed the coil to check the 14Mhz band which was found to be below 1.5:1. I could have tuned 14Mhz first then used coil to tune 7Mhz band, but for now, its good for me!. 

Happy with this experiment. I will be trying it at few places in near future.

Here is a short video of this setup and few photos.

27 turn loading coil is wound on a 2inch PVC pipe former with 1.6mm enameled copper wire. 

When the Coil is  removed from the PVC pipe, it slightly expands to ~55mm diameter. I then used few pieces of blank PCB material(without copper traces) and glued it at two places to hold the shape. 

Youtube link is as follows: https://youtu.be/T8sd8qcXF_M

What is the difference between AHVD and vertical? : 

AHVD is as name suggests, asymmetric vertical hatted dipole. In this system, one leg is longer than others normally by a factor of 1.5:1 ratio. i.e. vertical leg of the dipole is approximately 1.5 times lengthier than horizontal legs. Horizontal leg is for capacitive loading and hence the name "hatted" (hat is at the bottom instead of at the top here).

Vertical has two popular varieties, with elevated or ground mounted radials. AHVD may seem similar to elevated vertical, however, traditional elevated radial system is of almost similar length as of vertical radiator. I have not done any study of practical signal strength difference between two, as I did not had both of these at the same time. However, I feel both above have their merits and draw back being sometimes one may feel elevated legs/radials are obstructing and they require tuning carefully to a band.

Ground mounted  vertical requires many radials as possible, and efficiency depends on soil condition a lot more than elevated one. However, because the radials need not be of quarter wave, it has advantage of carrying bunch of wires and laying on ground.

 

Trial with portable Vertical antenna (AHVD) system and Fiberglass material for dielectric support

Disclaimer: I am no expert in the field of dielectric effects on Electro Magnetic Waves and antennas. This article is just based on my experimentation to find suitability of antennas/support for my QTH requirements. Also, no commercial interest is claimed for the source materials/brands mentioned in this post.

Recently due to Covid19, working from remote has provided some advantages to me in having some quality time with family and also to spend some time in the radio hobby. In last few years, I had diminishing interest in radio from my home QTH at Bangalore due to nearby obstructing buildings and also due to heavy morning QTH conditions. However, I tried to keep some interest alive in radio during contest seasons by mostly travelling to my native QTH near Mangalore and setting up some station there. 

Many new challenges there prompts me to try new ways to get signal out whatever little extent it may be. Thinking portable is a new mantra I am adapting to have some pleasure operating from places I go. One key challenge always is easy to carry and setup antenna system. Ofcourse, if NVIS communication is the purpose, simple low dipole/inverted Vee would be the best bet. However, for DX, it is low angle of radiation which is of particular interest to me. Only two ways to get such low angle radiation, one by having high dipole/yagi of some sought or secondly by deploying an efficient vertical antenna. During CQWW SSB Contest 2020, I had tried my hands at 4NEC2 antenna design software and deployed antennas at beach location with little success due to practicality of nice looking designs!. Making deployment simpler and repeatable was key to success. This year during 2021 CQWPX CW contest, I learnt that even simple vertical system can get me a bunch of QSO count required. However, it was not efficient and repeatable due to ground/elevated radial support system. 

Checking internet, I came across designs of Asymmetric Hatted Vertical Dipole(AHVD) from KX4O (hamradio.me) and also commercial bravo antennas from N6BT etc. Designs looked very similar to elevated radial verticals we tried earlier, but with better supports and asymmetric loading technique. Due to lack of telescopic tubing availability, I tried to make prototype vertical using prolite tripod, kitchen cutting board, aluminum 3/4inch angles, caperlan fishing rod and bare copper wires. Just by adjusting lengths of wires as per KX4O design  one could easily change bands. I made another small improvement to move down to 40mtr with ~12uH coil (adjusted by trial/error) on 20mtr dimensions as well. Though I could not make much contacts due to placement of the prototype antenna, I got some 20mtr CW DX contacts.

This is a great systems to try during field days and other portable ops for sure. Attached are few snapshots and a short video on the same. 

Antenna at ~4ft level            Loading coil between left leg and top vertical. ground only connected to                                                  right leg. Adjust the inductance ~12uH 

I tried this antenna only for 20mtr with 6.07mtr vertical wire and 3.96mtr each horizontal wire (adjusted a bit to get lower SWR). 

Following are few snapshot of my LCR meter showing reactance change with few common materials we use a support mast/junctions etc. I measured these inductive reactance changes for 1/2 inch dia loop with 4 windings.

              AirCore                               Aluminum tube                                 Caperlan 5mtr pole

Calcutta House Fiberglass tube        Kitchen plastic cutting board    12m Fiberglass Spider pole

I felt, Caperlan pole is very close to spider pole in dielectric characteristics. Though it is very thin at the ends, bare copper wire of 1mm dia will stand straight without any issues for portable setup. For horizontal legs, I used 3mtr poles. I tested effect of Aluminum just as a conductive material reference.

My inductance measuring loop over the fiberglass material.

A short youtube video of this setup is available at https://youtu.be/0F2CoAb8DQM 

Cheers

DE Kiran VU2XE