Will a long wire antenna work for bitx40 7mhz 7 watt ham radio? - Page 1
Forum section: electronics / rf-microwave

author: raspberrypi
author_rank: Frequent Contributor
timestamp: March 31, 2017, 11:23:54 pm
content: I currently have an 62' foot long wire with a 9:11 balun/unun toridal wire balancing thing on it. Its horizontal about 20 feet off the ground, with a 3' ground rod at one end that hooks to the balaun when it goes in the second story window. At 7 watts am I going to have SWR issues? I forget what band I made the antenna for ut I listen to 1-30mhz and that was the best length for the SDR.

author: KD0CAC John
author_rank: Frequent Contributor
timestamp: April 01, 2017, 01:04:01 pm
content: A number of issue's , 9:11 is not correct ratio number maybe you mean 9:1 ?Then how is it built , it can not be both a balun = balanced to unbalanced feedline or ladder line to coax , unun balanced to balanced ladder line to ladder line normally a 1:1 choke .An ideal hight of horizontal antenna is 1/4 wave of the lowest band of interest .Depending on soil , generally never less than 8' in ground and connected to all other house shack / grounds ." At 7 watts am I going to have SWR issues? " If your listening , where does the 7 watts come from ?And SWR has nothing to do with wattage output .Generally for listening , there is not much difference in length , no way to tell at this end what you mean by ut ?Also length of antenna has to do with frequency / band , nothing to do with what radio , mode SDR .So for some beginning suggestions 1-30mhz the lowest band 160m = 1/4 wave in length & hight - this is difficult to achieve for most unless your in the country and room .Since your just listening and if you have the room = min. 1 wave length long if not multiple wave length long and about 6-10 off ground , you may want to look into what's called a beverage antenna , one of the best for low noise & low band 40m being on the high side .Which may require a 2nd antenna , for 40m & above ?But for listening with SWR , most anything will work 20-30 feet long & high fed at one end into shack , using a non-inductive resistor about 10K ohm for static , noise , lightning protection / ground connected .

author: vk6zgo
author_rank: Super Contributor
timestamp: April 04, 2017, 11:19:40 am
content: The OP wants to use an existing receiver antenna, which he put up for use with his SDR, with the bitx40 7MHz radio transceiver.At 62 ft, the antenna is resonant at 7.54MHz, which is a bit above the Amateur 40m band.It could be fairly easily "fiddled" into resonance, by adding a bit of length, but the problem is that, in its present configuration, it is an "end fed" halfwave, which has a high input impedance that is not easily matched, even using an Antenna Tuning Unit.I would suggest you break it in the centre, add the extension on both ends, & feed it with coax.

author: vk3yedotcom
author_rank: Frequent Contributor
timestamp: April 04, 2017, 11:31:39 am
content: Breaking it into a coax fed dipole (roughly 33 ft a side) is a good approach.But if for some reason you can't do that then 62' is near enough to an end-fed half wavelength. You can use a simple L-match to tune it in.This is an example contact when using a similar antenna and a Bitx40A suitable antenna coupler is described atUnlike a fixed 9:1 it can match a range of impedances so the antenna length becomes less critical.