Monday, July 15, 2013

DU1FV 23cm YAGI from a CDR-King tv antenna

DU1FV 23cm YAGI from a CDR-King tv antenna 

Now that you have got a 23cm radio, you need an antenna. 

Unfortunately, they are quite rare and if available sometimes expensive.  Here are some photos of a prototype yagi I made from parts off a P200 HDTV antenna from CDR-King.







Below is the original P200.00 HDTV antenna. It is a relatively compact antenna.

There is a lot of element (~1/4 inch Al rod) material so it is perfect for protoyping.

We proceed to strip the original elements from the boom,




The design of this antenna is based on the method of perturbation.

The following steps are followed:

0. Mount the boom securely.
1. A driven element (DE) style is selected.  In this case I choose a double bazooka style of element.
I made it out of 3/8 copper tube from a ref supply house.I split the center and inserted the foam insulator with center conductor from a piece of 50 ohm foam type coax.  To make it a bazooka, I soldered the center conductor by flaring the strands so they touch the rim of the copper tube at the ends. See picture below.



2. The DE is cut so that it is resonant at about the target frequency and is tuned as a simple dipole directly fed by coax via suitable connectors.




 It has to be mounted on the boom when measuring swr.
It took me three tries before I got an acceptable reading, about 1: 1.8 to 2.0 I used a vhf-uhf cross needle swr meter by Daiwa. It is not calibrated for 1200MHz But it reads 1200mHz energy quite well (too well  in fact: ten watts wold read 25w) and is ok for relative measurements for swr.
 3. A field strength meter (FSM) is devised from a common analog volt ohm meter. If you tx with your portable with rubber duck antenna next to it there should be some reading while it is in the AC posiition.  It does not have to calibrated because it is only use for relative measurement.
4. A wood chopstick is used as a temporary element support and one element of the orig tv antenna is cut to about textbook length of a dipole plus maybe ten percent. Use masking tape to attach the element to the chopstick.
5. Position your FSM along the book till you get a reading when you drive the DE.  Then slowly position the first parasitic element on the chopstick in between the FSM and the DE. Obeserve if signal goes up or down as the field is "perturbed" by the proximity of the parasitic.  Compare the reading with another one taken with this time the parasitic in the "back" of the DE (which puts the DE in between the FSM and the parasitic.  since we have cut roughly long w the behacior we are looking for is tha of a reflector.
6. Generally if the signal reading goes up when the parasitic is between the FSM and DE, the parasitic is "short" and behaves more like a director.  If on the other hand, the signal decreases, it is acting like a reflector effectively shielding the FSM from the DE.  If this is the case, yo will generally find that if you put in the back of the DE, the FSM will read slight ly higher compared to one taken with the parasitic removed from the vicinity of the DE.  You will now have a feel if the parasitic is long or short.
7. Make another parasitic, one slightly shorter maybe by one half mm. Try to replace the old parasitic with the new one.  If it sit still behave as a reflector , that is good. If it is a better reflector (measure FSM) let it be the new reflector.  You can now grind down the first parasitic you made to make it shorter by half an mm. then replace the mounted one with the newly shortened one and see the FSM reading.  By now you should get the drift.
8.  There will come a point that as you shorten the parasitic it will become a "director" and you can confirm this by placing it in between the DE and the FSM the signal should go up dramatically.
9. The same process of slowly cutting and grinding down the succeeding parasitics will yield a quite decent antenna. as long as you are careful with FSM relative measurements.  also check the swr now and then.

Remember to always compare reading BEFORE and AFTER a CHANGE.  And follow the EXPERIMENTORS CREED: " CHANGE ONE THING AT A TIME"



10. At one point you may even feel that your driven element is too long or short as swr will vary as you add elements.  In my prototype I did have to change the DE length just once after adding the fourth parasitic and the swr kick back to acceptable level.







    This in a nutshell, is the process of perturbation. Unlike blindly following published design specs, it "automatically" takes in to account element diameter, height above boom, diameter of boom etc.  It is a bit like analog computing.  The principle is not my own. I first read about it in a ham magazine during the 80s.  I am not sure but it was probably QST and the author did it for a uhf beam.

Notes

    There are existing evenly spaced holes corresponding to element spacing of the HDTV antenna. I decided to stick the original spacings and do element length tuning only, so I can use the existing holes.  There might be more optimum spacings but I did not want to weaken the boom too much by drilling more holes through it.  Since this is a cheap HDTV antenna, the boom is not too sturdy.

Enhancements

     This is a prototype and can stand some improvement like enclosing the entire driven element in a waterproof radome of some sort, but I will leave it up to you to think of enhancements.

73 Ramon DU1FV

















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