[PVRCNC] n4cwz antenna question

Sun Dec 18 11:43:52 EST 2022

Hi Dave,

Sorry to hear about your RFI issues with OCF dipole.  I’m not surprised since it’s a non- resonant antenna and relies on the feed line to be a portion of the antenna.  You’ll find that changing the length of the coax feed line by 1/8 wavelength on 80m and your SWR and RFI will probably change.  

I’m a fan of resonant antennas.  You can more easily control RF on the coax with a good center balun and an additional choke balun along the feed line if you find you’ve got RFI from common mode RF currents on the coax.  But, most resonant dipoles don’t have issues.  Except 80m dipoles tend to have a narrow bandwidth(~100kHz) compared to the 500kHz 80m band.   When you get outside that 100kHz window you start getting RF on the outside of the coax because the antenna no longer matches the 50 ohm coax impedance.  Being the CW guy you are, cutting the antenna for low end of 80m is probably what you want.  It will just have a high SWR in the SSB portion of the band.  

On 40 m, you get a little more bandwidth, maybe 150-175kHz.  So, you still may have a reasonable match into low end of the SSB portion of the band with the antenna cut to cover the low end CW portion of the band.  

On the RG8X power,  I think you will find that at the lower frequencies of 80 and 40m you will be fine to 1200-1500 watts with low SWR.  At higher frequencies above 14-15 MHz the power rating drops to 1000 Watts and lower in the 30-50mHz and up.  You can find the power charts/tables on the manufacturer’s sites like Belden and Southwire.

I prefer to use an RG213 or LMR400 equivalents rated for ground burial(ie buryFlex).  It has lower loss per 100 feet at high frequencies and holds up better to UV and ground contact.  It is heavier.  But, if you are supporting the dipole in the center it’s not a problem in my experience.  You can always start with RG8X since you have it, then replace with LMR400 later.

I have a 40/20/10m fan dipole that I use here and it works well.  Takes a little effort to tune because there is some interaction between bands when trimming the antenna.  Keep the band elements about 6 inches apart.  I use some small 1/2” PVC to separate the wire elements.  I’ve got 6 of these space about 10 feet apart across the ~66 foot dipole and use paracord to extend the shorter 20 and 10m elements to the outer spacers.  

Hope this helps.

Merry Christmas,
Eric - NR4O

Sent from my iPad

> On Dec 18, 2022, at 9:52 AM, Howie Hoyt <howie.n4af at gmail.com> wrote:
> 
> Hi Dave -
> A little concerned what led to the problem ? Did it originally NOT have RFI
> ?
> RG8x should work- i have some big hardline u can have but difficult to
> transport.
> 
> Sorry to hear about your knee.
> 
> Vy 73, Howie N4AF
> 
>> On Sun, Dec 18, 2022 at 9:35 AM dave collins <davetct at gmail.com> wrote:
>> 
>> Hi folks,hope everyone is well.  Well i don't know what I did,but my ocfd
>> has become unusable because of rfi on 40 and especially 80. Thought It got
>> into my pc,but it had gotten into my ups!  I'm wanting to put up a fan
>> dipole for 40 and 80. Ordered a balun,so one common feed and I think I will
>> be good. Presently I'm using rg8x.  I think its too small for running more
>> than a kw. Is it??  I've looked and have seen conflicting info on the web.
>> The reason I'm using that is its lightweight . Everything else is lmr 400.
>> So,if yall think its too small,what coax do yall use coming from a dipole?
>> About 150 ft run. I gotta get busy on this as Im having my first knee
>> replacement on the 13th!  thanks and 73 dave n4cwz
>> 
>> --
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