Sunday, May 14, 2017

Projects - Inverted V antennas for the low(er) bands

Choosing a /P antenna for 80m

My favourite antenna when working /P is the end fed half wave, set up vertically. It is easy to set up and works well with reasonable DX performance. It will not beat my portable hexbeam when working DX but that one requires more time, material and open space to set up.

The EFHW vertical becomes a challenge however on the lower bands. Now I did get myself the largest Spiderbeam pole - so a vertical half wave for 40m is an option - but there is a limit and 80m will definitely not fit.

One option then is to use a quarterwave vertical. However this requires radials if you want the majority of your signal to be radiated above ground. I developed a quarter wave for 80m with four elevated radials and it works but is quite a hassle to set up. Working /P you don't always have room for four elevated radials.


Another option is to use the EFHW sloping. That is a good option and I have used it on numerous occasions. If the support is tall enough to allow for an angle around 45 degrees the antenna still has a low angle of radiation but also a lobe at a high angle. That makes it a versatile antenna. It has some directivity - in the direction of the slope. 

The challenge on 80m is that the EFHW sloping still requires a tall support for a reasonable (steep) sloping angle otherwise you will end up with only the high angle radiation. 


Inverted V antenna

Another option - using a single support - is to use an inverted V dipole antenna. For 80m I find it a convenient antenna to set up and it comes with a unidirectional radiation pattern (in the horizontal plane) that fits my needs when I am "being chased" with chasers from all directions.
The inverted V has a high angle of radiation. On my 18m pole it very much resembles a sloping half wave dipole of the same length. The main difference is that the sloping wire will have 6dB F/B ratio while the inverted V is omnidirectional. It is very much up to what you are looking for.

Practically when working on 60m or 80m (or 160m) - for more than a few QSOs - I choose the inverted V antenna. When my tallest support still was only 12m I also chose the inverted V as my default antenna for 40m.  


For my inverted V antennas I developed a universal center connector that fits on the fiberglass poles I use. It is a simple component made from 40mm PVC that allows me to connect various wires to a SO-239 connector. The reason I designed it with changeable wires is that it allows me to make different combinations like 40m and 80m inverted V's on one pole fed by one coax cable. 




The tie wraps on the sides are used to keep the cable tension away from the soldered cable shoes - otherwise the shoe will certainly break in due course. The hole through the center allows it to slip over the top segment of my fiberglass poles.

I developed a couple of these center connectors so I can set up different antennas at the same time. We use them for example with the YNOMY DX Group during the PACC contest when we set up three different inverted V's (40/80/160). We then have the 80m and 160m on the tallest mast together, being fed from one coax cable.

For each inverted V I have two separate dipole legs. Each dipole leg is cut to the right size and features a cable shoe as visible in the first picture. To hold the wire I have constructed a cable spool consisting of a piece of PVC (40mm) with two end caps. On each end caps I have glued a piece of wood and through the end caps and wood I have put a piece of threaded wire. 


40m wire spool (one leg)
This spool holds the antenna wire plus a long enough end of tension string. I find it works rather conveniently in that I can just stick the spool in the ground (slightly under an angle) where it unreels while I extend the mast. Once I am happy with the position of the mast and antenna I push the spool further into the ground, stopping it from turning. This way I do not need pegs or anything else to set up this antenna. 

Below you see an impression of the antenna with four legs - dipoles for 80m and 160m - ready to be deployed.



I just stick the spools in the ground in the direction I want the leg to go. Then I extend the fiberglass pole with the spools unreeling until the mast is fully extended. Then I pick up each spool and place it where I want the end point to be. The advantage of the spool unreeling is that there is only a slim chance the legs get entangled even when you have two dipoles on one mast - the wires are kept under some tension the whole time. 

5 comments:

  1. Lars en beide Marcels, veel succes met de LX wff tour. Ik hoop dat jullie veel verbindingen zullen maken. 73 Hans. PE1BVQ

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  2. Uiteraard bedoel ik Marcel en Marc.

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    Replies
    1. Bedankt Hans. Het was is een mooie belevenis. Er volgt nog een post als we bijgekomen zijn en de administratie bijgewerkt hebben.

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  3. How pole handles weight of center piece and whole antenna? I have Spider Beam fiberglass pole 12m high, and I do not attach 80m inverted V to the top, but maybe two meters lower, as top bends a lot. I use center piece that seems lighter than yours.

    I am considering adding BALUN and main concern I have is weight.

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  4. Dobre vecer Pedja,

    Weight is an issue if you want the center up high. The hole in this center piece makes it slide down to approx. the end of the top segment of the Spiderbeam pole. It bends a bit but nothing to dramatically (unless you have strong winds of course). It is not completely vertical - so maybe if I put it down 20cm lower I will only lose 10cm, because the pole will be less bent. But I think the net result will be that the center will be lower - even if it looks better.
    You have to guide the coax along the pole btw.

    If you think about a balun you might consider attaching it a bit lower after some coax length. W2DU style chokes are not too heavy and might be an option to use from where you fix the center (2m below the top). I use them a lot.

    I have used the 12m HD version of Spiderbeam for a 40m inverted v and it worked nicely. For 80m I use the 18m pole by Spiderbeam. It is also the HD version and in many ways resembles the 12m pole. I also slide the center to the bottom part of the first element. I used a Spieth mast before that and it was less thick. The top 2 segments were unusable for an inverted v. The HD masts from Spiderbeam are much better.

    I hope this helps you somehow. Good luck with the experiments.

    73,
    Lars / PH0NO

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