Australian Ham Radio Discussion Forum ( AHRDF )

In parallel (pardon the pun) to developing a new multi band dipole type wire for my QTH I want to make a portable vertical for use on field days.  

I was hoping to use a nice telescoping 7m fibreglass rod I have spare as the main vertical support and this got me to thinking...

When building a vertical like this is there any reason why you cannot use a spiral wind on the wire?  It would seem that way I could put up a 80m 1/4 wave by wiring the 20m element onto the 7m pole... right?   Is there a reason this doesn't work?  It seems to be the approach used by most CB whip antennas...right?

Ideally, what I want is an easily erectable vertical usable on field days that I could use for 80/40/20 via a tuner.  This would mean a 9:1 UNUN and 1:1 RF choke at the bottom to stop RF cmoing back into the tuner, an earth stake and a bunch of ground counterpoise wires.    Hopefully have a 20m long wire in the air (albeit only 7m up) should allow the tuner to work on these bands at least...?

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OK - before he jumps on me I just found his link and am reading it now...

http://www.vk4adc.com/web/hf-projects/45...-whip-calc

OK - so it looks like the major trade-off is bandwidth huh. I wonder though, would a tuner be much help in that case?

Second trade off is the number of turns - 100m of wire on a 7m long pole = about 3000 turns! Ouch, good luck home rolling that beast...

There have been a number of articles in the past about using a "Slinky" as an inductive loading for a HF vertical. 
(A Slinky is a toy precompressed helical spring invented by Richard James in the early 1940s. ... Betty was dubious at first, but changed her mind after the toy was fine-tuned and neighborhood children expressed an excited interest in it.)
Probably not what you were thinking of but it immediately came to mind with your description.

My approach with the HF whip on the caravan has the same basics as you mention : a 7M squid pole, wire, a multi-tapped un-un and an RF choke effect plus an optional tapped inductance wound on electrical conduit for 80M.  In my case, I have the antenna wire go up through the centre of the squid pole and out a hole in the base mounting arrangement. When the squid pole is extended, the wire is pulled through the side hole and up into the centre of the fibreglass tube. The wire is pulled out the hole as it is disassembled and then just wound up and left at the base for travel. The wire terminates in an insulated croc clip so it can't pull all the way through the side hole.. That's the antenna wire part.

That croc clip is connected onto one terminal of an UN-UN box with terminals at 4:1, 9:1 and 16:1 and the one to use for any given band is determined by trial and error during testing.  A BNC socket on the other side plus a "ground" terminal  completes the box. 

The coax choke is a short length of RG58CU fed through a series of the ferrite tubes from Jaycar - I think there are 5 or 6 in the packet - and these are a good fit over the RG58 outer.  The whole ferrite 'stick' is then heatshrinked into a long tube for protection against damage.  There is a BNC male one end to connect to the BNC on the UN-UN and a BNC female on the other for the incoming feeder.

The external loading coil is seldom used but is something like 100 turns of tinned copper wire with taps about every 5T and the final tap position (where the antenna wire croc clip goes)  is determined by trial and error.  The 'cold' end is a stiff wire with another croc clip so that it can go to the best-results UN-UN terminal.

Does it work ? Yes, I have worked quite a bit of FT8 on 10MHz (where it is approximating resonance as a quarter wave vertical), more on 14 and 18MHz.  It does load up on 7MHz and I can hear and work lots of interstate signals on that band.  The raw wire antenna is almost useless on 3.5M, better with the base-loading inductor.  I use my caravan drawbar mount as the "ground" and it all goes together quite smoothly.

I have to admit to having a couple of wire radials to be able to use too : a 7M wire equivalent to the actual antenna wire for all bands plus a 19M long one for 3.5MHz use only. More croc clips to add on as required !

The in-line LDG tuner does all the matching magic stuff to present a reasonable load impedance to the transmitter on any band.

Have I spent a lot of time experimenting with this ? You betcha !

And... if you are wondering why mostly FT8..  You don't have to have the receive audio up loud nor do you yell into the microphone so the others ( caravanners) around you don't even know that you are on-air.
 
Deception : I often fly a Eureka stockade flag from the top of the squid pole, held there with a couple of fabricated plastic clips, so most people think it is purely a flagpole

PS..  a couple of hours later :  have a look at https://www.vk4adc.com/web/45-hf-project...-revisited too.  It is not the latest version of the squid pole adaptation but may assist with your contemplations.
 

Have a look at this article.
http://www.dj0ip.de/vertical-antennas/si...-vertical/
It might suit you, it might not but its worth a look. There is some interesting stuff on his website.
Nothing ventured, nothing gained.

Cheers
Nev

My portable antenna is a 13.4m wire fed via a 9:1 UNUN.  I can use it as a long wire with the end tied on a tree or my 7m portable fiberglass mast ("squid-pole") or wound around the pole as a helical vertical.  With an external tuner I can use it 80-6m.
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Wayne VK4WDM