Distance to place chokes from feed point?

[cwguy]

I have a Bullet-8006-500 End Fed OCF from Palomar Engineering setup and would like to know what issues I may face if placing chokes at a distance closer or further than the recommended distance as given from the mfg.

My line looks like ...
ant <- (25 ft. rg-8x) -><- (50 ft. rg-213) ->(entry panel)<- (25 ft. rg-8x shack)->radio

8X runs down the pole, connects to 213 which is buried and runs to an entry panel, then it's 8X into the shack.
Would it be ok to add the chokes at 20'? The antenna manual says to install at 38' from feed point. I could run 40' of coax from the top of my pole but it's only 20' high and I'd be rolling up the remaining. Another option is to place them just before the entry panel into my home.

Thoughts?

 

[Ham4CW]

There are two issues with this antenna. One is that it relies on the length of the feeder (from the feed-point to the choke) as part of the overall length of the antenna, and as such any changes to this length will affect the resonance of the antenna.

So if you move the point where the feeder is choked the various resonant frequencies will move up or down depending upon whether you make the length (from the feed-point to the choke) longer or shorter. Also, if you were to move the choke too close to the antenna feed-point (you mentioned 20') then the resonances of the various bands may well move so far as to render the antenna pretty well useless for its intended purpose.

Many of the Off Centre Fed (OCF) antennas I've seen generally use either a 4:1 or 6:1 matching transformer, which loosely brings the impedance with the range acceptable to the 50 Ohm feeder. If the resonant frequency is shifted too far from its design coverage, then the impedances at the feed-point will be altered also, possibly to the point where you will not only have a very poor match between the feeder and the transformer (balun), but you could also end up with large power losses within the transformer core and even risk burning out the balun/transformer due to the very large voltages that may result.

If you rolled up the remaining length (as you suggested), then yes, you could have 40' of feeder between the antenna feed-point and the choke, BUT, because the feeder is coiled up it will no longer have 40' of electrical length as required to make the antenna resonant on the various bands it was designed to cover. It would be as though you had fitted the choke say 30', or even 25' away from the feed-point. By coiling the feeder you would in effect be shortening one half of the antenna.

Another problem with the feed arrangement is the fact that it is using the feeder outer shield as one half of the antenna system. This means that the outer shield/braid of the feeder is RF 'hot', and because the point at which it is choked is acting as the end of the antenna 'element', then bringing this close to anything conductive will again greatly alter the resonant frequency of the antenna. It's a bit like building a half wave dipole and then running the one half of the dipole along the ground!

A 'traditional' end fed half wave only requires a small capacitance to allow the antenna to work, so you can run the feeder pretty much anywhere you like. A 'traditional' OCF dipole has the feed-point and the elements elevated and generally the feeder is choked directly at the feed point, so again with this antenna you can pretty much run the feeder wherever you like without affecting the antenna performance.

With this 'hybrid' of the two ideas however the feeder is no longer dead but instead forms part of the antenna, and ideally needs to be both elevated AND kept away from conducting objects (including the ground), in order for the antenna to function effectively. There was another antenna that used a similar idea produced by RadioWorks (their Carolina Windom).

Yes, you could move the choke point, yes you could run the feeder anyhow you like, BUT at best it will end up being a compromise running with reduced performance, and possibly it might also affect the matching so much that even the auto ATU in your transceiver might fail to obtain a good match. And even if the tuner does achieve a 'good match', if the layout of the antenna is compromised so will performance too.

There's some good info on 'traditional' OCF dipole designs here: https://www.pa0fri.com/Ant/FD4/fd4eng.htm

Hope that ramble helps.

73, Mark...

 

[LW1DSE]

My knowledge about industrially manufactured antennas is quite low or null. From the very start of me at radio in 1987 when I got my LW1DSE call, I used to build my own aerials, and even my own rigs. However, many commercially available are based in the ever-deseiver adverticers. Anyone that has studied a minimum of Physics know that antennas don't gain anything, their gain is only relative (from here the use of dB notation) over an isotrophic radiator (a conductive sphere), or a simple dipole. In fact, any antenna has looses: both resistive (in series because of copper or aluminum electrical resistance) and parallel conductance to ground via non perfect isolator(s).

Said this, and ignoring me about the aerial you are asking for, it seems too complicated and expensive, requiring several adjustements, and then, critical. More over if you will use it with semiconductor power stage, some times delicated with SWR. It is more easier and cheaper to build an antenna for your own, and the satisfaction when results appears.

May I suggest one of this easy to build aerial? Take a common AC wiring of such long as half wave of the lower frequency you want to TX&RX. Get two isolators for dipole, here in Argentina are called "aislador huevo" (egg isolator, pic below).

Thus, own a ferrite core of AM transistor radio or a large toroidal core. Build on it an autotransformer of, say, 20:1 turns ratio of wire of sufficient wire diammeter to support RF currents. Turns may be a bit separated in order to reduce intrawinding capacitance. Hook the coax to the smaller winding, live to tap and shield to shorter end. This will cause to be a step up autotransformer, both voltage (20:1) and impedance (20²:1 or 400:1 ), giving 400 × 50R or about 2KR to ONE END of the dipole. Ground the outer conductor and autotraffo common return if possible.

Thus, this model will operate at, say, half wave at 80m (40m of continuous wire), a wave at 40m, twice wave at 20m, etc without changing anything. The lobes modifies as the size of the wire changes respect to the wave long. But in any instance you will comunicate perfectly without headaches.

The remote end of the wire remains isolated, as far from conducting objects as possible.

Cut small pieces of wire, longuer first, shortes after to find proper resonance. During adjust, use low power to avoid core saturation if SWR is large.

Finally protect the autotraffo using a discarded mermelade glass jar, open end looking down.

I used this topology some years ago when I still live at dad and mum's house with my FT767. The wire was located diagonally over the street, and remote end in a telephone pole.

Osvaldo LW1DSE from Argentina.