Making antenna adapter for portable grundig SW: coax to wire? - Page 1
Forum section: electronics / rf-microwave

author: Beamin
author_rank: Super Contributor
timestamp: May 31, 2018, 08:30:43 pm
content: I have a Grundig SW with the external antenna input that looks like a mono head phone jack. The last adapter I threw together used a mono headphone to F adapter. That goes into a 6" long (75 ohm was cable that the cable company left after an install) where the shield is pulled back and the center conductor is soldered to the antenna and shield goes to another wire for ground. The antenna and ground wire are the same 22 ga stranded wire.Does the impedance matter so much? The radio is 50 ohms (probably) then 75 for coax then what ever the wire is. My understanding is the impedance of a SW antenna comes from the shape and height over ground and not the wire? If tis was a VHF/UHF antenna what would be the impedance of this wire 300 ohms? 460?I also have a 10$ Chinese ebay balun that's 9:1 or 11:1 which ever the standard is. Its just a little ferrite ring in a plastic box with magnet wire on it can't tell how many turns. Would that be important to put on it? Since the box had no ground I hooked the ground wire to the shield side right in the balun case.When I had my SW SDR hooked up to a long horizontal piece of wire from the roof to the tree I used this balun with it grounded into a ground stake. That would be the same for this right?So is this the optimal way to build this or do I have it all backwards? I have limited access to messing around with the antenna so I want to build it right the first time.My other idea is to make an indoor antenna with the wire wound around a card board tube or something where you can handle the antenna as a complete unit.

author: tkamiya
author_rank: Super Contributor
timestamp: June 01, 2018, 02:57:10 pm
content: Nope! Impedance does not matter at all!Firstly, your antenna input on Grundig is not nominal 50 ohms or 75 ohms or anything. Plus, it changes with frequency.6" long of coax is negligible relative to wavelength of the frequency. Besides, antenna itself is not resonant, so its impedance is all over the place.I wouldn't even bother with balun.With this many things NOT formalized, everything is experimental. By the way, my setup is similar to yours. 20 meters of wire from a tree to a house. Same wire is used from there to near the grounding rod (bonded) and transition to coax. About 30 feet of coax goes from there to my receiver. It would be awfully inefficient for transmit but for receive, it has been working wonderfully.I don't know why baluns got so popular.... It is a device to convert BALanced something to UNbalanced something. Then some will do an impedance transformation. In your case, all of it is unbalanced and impedance is undefined. So it is not necessary.

author: Beamin
author_rank: Super Contributor
timestamp: June 02, 2018, 07:35:51 am
content: Quote from: tkamiya on June 01, 2018, 02:57:10 pmSo that my second question. When I tried to use that antenna (Same set up as yours 30 ft coax to a window and balun) to transmit 7 watts at 40 meters I didn't have any luck making contact. At the time I didn't have an swr meter so I have no idea how much I was losing. So if I want to transmit I have to pay very close attention to the length and for the most part use 1 antenna for each band, no universal antenna for transmit unless it just happens to be a 1/4 multiple which is rare?Soo really the only way to know if find a length as best you can just play around with it. Doing short cuts with math isn't that useful.

author: tkamiya
author_rank: Super Contributor
timestamp: June 03, 2018, 01:49:20 am
content: I've done that, too. I am in Florida, USA. With this kind of antenna and 10 watts, making contact is REALLY hard. Having a huge loss caused by mismatch and a compromise antenna doesn't do any good either. Receiving works well because Ham grade receivers are hugely (and often overly) sensitive. QRP (small power contact) is nice but only possible if your location is excellent or your antenna is superb, or you are just lucky.You may have a different experience if you put a match box right at the bottom of the antenna (before coax), have a good ground, and have a good match. Even then, low height and compromise antenna will hinder you.There are two types of antennas. Resonant antenna and non-resonant antenna. Dipole is a resonant antenna. One length of antenna works on one band and odd multiple. Say you make an antenna for 7Mhz band. It will also work for 21Mhz, but not on 14Mhz. What you have is a non-resonant antenna. One antenna works on all bands PROVIDED you have a suitable matching network. This network needs to be BEFORE coax because mismatch in coax is awfully lossy.One more thing.... If you are using more recent transceivers, transmitter will "fold back" meaning power down if you try to transmit into less than good antenna. Otherwise, your rig can overheat and pop expensive transistors/FETs.My suggestion is, to make a dipole and invest in an inexpensive SWR meter. 7Mhz and 14Mhz are quite active, so you can choose depending on your licence and location. Get a good match, say less than VSWR 1.5. You really don't need balun. I know the theory but I've done both and I can't tell any difference. How we do things makes our antenna really not so well balanced anyway.I've been a ham for 40 years with inactivity in the middle. Please feel free to ask me anything and I will help when I can. I believe there are more hams here as well.

author: Beamin
author_rank: Super Contributor
timestamp: June 03, 2018, 03:07:06 am
content: So this is what I always wonder about: Can the ground make things worse? Should the ground wire connect to the balun at the beginning of the antenna or should it be closer to the radio? Provided the ground wire is unshielded 20 ga that goes to a copper pipe hammered into the ground. Would there be any advantage to using the center conductor of extra coax from your ground? I have tons of extra 75ohm cable tv coax laying around.